"The reason theists are tirelessly trying to convert the populace is because they are motivated purely by the desire to save our immortal souls."
This proposition strikes me as unlikely - though I might be wrong.
When people engage in large-scale coordinated activities to counter what seems to me to be normal, I get suspicious. So rather than confront THEIR issues directly, I prefer to try to surmise what really motivates these people. For example:
Why would people, decades after the science was "in", continue to maintain that smoking was harmless.
Why, when prohibition, "Just Say No!", and the "War on Drugs" have been demonstrated to be abject failures, do people continue to support and pursue these strategies?
Why do people deny climate change science?
In these examples, I could suggest, "follow the money".
But, back to the point, why do theists care what we believe? What motivates THEM (if not altruism).
I don't think there is "a" reason for theists to try to assimilate others, or a single motivation for their worldviews and actions. I suspect it is an ugly tangle of issues that holds them in place. I won't claim to understand them all, but here are some I am pretty sure contribute:
Fear of the unknown. For many, a BS answer is better than no answer. Hence, the "god of the gaps" rationalization.
Peer pressure. It's scary to stand up to, or even question, family, friends, coworkers, employers...
Security. Belonging to a group is, I would suspect, an evolutionarily stable strategy and thus would be deeply ingrained.
Brain wiring. If I remember some of the neuroscience I have read, the religion centers of the the brain (most notably, I believe, the temporal lobes) give quicker and more powerful signals to the central brain areas.
Brainwashing. Many people have grown up within a given religious system and have psychological and physical difficulties even comprehending the possibility of a worldview without supernatural elements.
Disinformation. There is no lack of bad information being given to religionists by clergy, family and others who wish to maintain the numbers in their socio-political sphere of influence. I have yet to talk to a religionist who didn't have a totally wrong understanding of evolution, for instance. With things like the creation museum, televangelists, Family Radio, and all sorts of other outlets spewing lies and ignorance, the raw materials for conversion to a reality-based worldview are inadequate.
Lack of critical thinking skills. The necessary skills needed to break free from religious BS are simply not known to them. Removing a screw is much easier if you have a screwdriver.
I'm sure there are more. This is why it is so tough to respond to them effectively. There may be some who honestly have compassionate motivations, but I think that religions squash much of that by propping up selfish motives.
They care what we believe because our differing worldview threatens the complex-yet-delicate structure of their existence. I think they sense that.
(Sorry if I rambled a bit. I think it is time for me to hit the sack...)
I think Bob hit many of the key reasons. The only thing I would add, perhaps under the "brainwashing" category, is that this is how religions are set up.
When man invented various religions as a means of controlling others, gaining political influence, gaining wealth, etc, a huge part of the objective was to indoctrinate others so that they could accomplish said goals.
No followers = no influence.
These less wholesome motives are masked by more altruistic veils, ex. saving souls, helping the poor, etc, so the rank and file buys in.
For that you would be looking to the Neo-Cons. Essentially they are just using the Christian Fundies to get votes so they can take the helm and propagate the politics of fear. They started with Reagan, and got good inroads with Dubya. Start with Leo Strauss, then follow down to the Krystols and Wolfowitz et al. They are only allies with Cheney and Rumsfeld.
None of those connections, however, explain the motivations of the fundies. For that just look to the millions made by televanegelists, as well as the power that goes with those positions.
I think there is a reason to assimilate others from the perspective of "the system" for the purpose of gaining followers just like a corporation seeks buyers. I don't think religious belief started out this way, as a means to fleece people of their money or property but as it grew it needed leadership and as leadership power and control increased so did their desire to control more people. Corporations use propaganda tactics to sell their product which of course is propagated to be better than that of it's competitors. Atheists are denounced by all factions since they wouldn't buy the product anyway (we make bad customers). Theist are in large part simply reiterating the ideology of something created probably as a result of an understandable ignorance of life, death the cosmos etc. but hijacked by those who's desire was not to benefit the people but to control them. Once in control of a largely illiterate populace, it becomes much easier to tell them what the bible says while inserting whatever dialog you wish so as to make them even more compliant and to then go the step further to command they convert others. In regards to the U.S. there seems to be a concerted effort to alter the perception of history (Thomas Jefferson was a christian, this country was founded on christian principles etc) so as to cast doubt on it and as we all can attest, religion thrives on the margin and the grey area. If they can muddy the waters enough the whole conversation becomes confusing to their flock which is exactly the way they like it. The flock eventually concedes that they don't understand but they know someone who does and that someone is a part of a system that is working tirelessly to exploit them. Factor in vast wealth, a political system which is now overwhelmingly christian and the stage is nearly set. Add into this mix a quickly growing populace of non-faithful who are always willing to argue and one only has to wonder when there will be a series violent backlash. Richard Dawkins stated that a fair amount of his email is hateful and or death threats, how long until they make good on the promise? I think it's safe to say that the U.S. is going to have to deal with some unsettling growing pains.
Sorry for mistakes typing quickly have to make dinner.
I think I understand where you are coming from now. You want to know how the anti-evolution movement has come into so much momentum. That was not at all clear from your original post.
For the most part you can just follow the money to the churches. Parishioners are often pressured to tithe 10% of their wages, often with the goal of that being calculated on gross although typically calculated on net. As much as all Christian sects might disagree, they will almost all set aside their differences to agree that science is becoming their nemesis. Even if they do not directly conspire to set-up the creationist lobbying groups, they will all contribute to that movement. Getting creation taught in the science class rooms gets them a lot of credibility with the current generation as well as maintaining inroads with the next generation.
Those who lead the lobbying groups are likely all believers themselves. Believe it or not, some of them are even extremely well educated. Kurt Wise has a PhD in geology from Harvard and he has made it clear that no matter how much evidence there is for an old earth he will always believe in young earth creationism because that is what the bible tells him. Obviously a guy like this can't make a lot of money as a Harvard professor, but he can make a lot of money twisting the science in every fashion possible to make it fit YEC. There are a lot of people who are very eager to hand over money to have 'scholars' exactly like him do exactly that.
So it's not just about maintaining church revenues, it's also about a desperate clawing at anything that will support their beliefs. Between the two, motivations are about as high as they can be.