The main reason is that Mars is a very unhealthy place, not just for humans but for all life. Forget the temperature extremes. The biggest problem with Mars for maintaining life is that it has almost no protection against radiation. It has no iron core like Earth and thus it does not have the magnetic field that protects humans from cosmic rays and solar mass ejections.
Read this article for more information.
Six feet of soil can shield against cosmic rays as can a few centimeters of water. Since hauling water to Mars hardly seems practical, unless an ample water supply is found on Mars, the "explorers" will have to be satisfied with living and exploring under the surface.
Also, the human body is so adapted to our level of gravity that almost absent gravity (as in the space station) damages bone density. In a large space craft that rotates, you can use centrifugal force to simulate gravity, but how do you do that on the surface of Mars? Not exactly a vacation and no place anyone would want to live for long.
Terraforming isn't practical unless there's a way to give the planet a magnetic field. Otherwise, the atmosphere will simply be lost to space.
What do you think?
Should my wife write a letter to NASA asking if they need someone to photoshop out the 'Mars' weeds, jack rabbits, and contrails? There is a distinct feeling that a future in state propaganda could be a growth industry...LOL
RE: "We have no need to have humans go to mars."
Humans, no, but can't we send Unseen?
In the movie 'Contact', one character tries to make the case that the '..best astronomers would be vampires!' Surely there must be folks that would see a permanent post on Mars as a great improvement over our common fair of crime, wars, politics, movie remakes, and 'reality shows'.
If it were not such a terrible investment to get there, I would pay $1.00 to see some CEO's abandoned on such a quiet piece of real estate. I think I can see the closing credits as the small speck of Earth, sets on the Mars horizon, and the line of sight microwave transmission signs off, to be replaced with white noise from the solar wind.
Oh dude I figured this one out a while ago, this is on my mad scientist to do list. The earth's core is able to produce the protective electromagnetic field partly because of the significant amount of friction caused by our relatively big moon's gravity. Mars' core on the other hand did not have the benefit of one big moon, so that and its low mass led to the cooling of it's core, the weakening of it's electromagnetic field, and the eradication of its atmosphere. What if we someday developed the technology to give Mars a big moon, maybe move Ceres into orbit around it, slowly add the entire masses of Mars' other two small moons to Ceres, and as a bonus water ice on Ceres could be shipped to the Martian surface to be used by colonists on the planet with a magnetic field and atmosphere!!! As for long term settlements, we could engineer people and biospheres adapted for life on Mars. It won't be in my lifetime, but it could happen.
Why wait? Maybe we could just capture Darth Vader's Death Star in a tractor beam to bring it to Mars. Assuming Scotty couldn't beam it from wherever it is to Mars, of course.
If our history has shown us anything we humans are not content in staying put for very long. I think we will be in space sooner rather than later.