Where's the money?
Exploration has always been driven by Man's insatiable curiosity, need for expansion and investigation. Colonization, on the other hand, has always been driven by profit and nothing else. America was settled because it was a land of huge resources and settlers could build a better life there than in their countries of origin. The "scramble for Africa" was driven solely by a desire to exploit the resources found there. And so on.
Mars, on the other hand, has little profitable resources to offer. Yes, Man could conceivably eke out an existence there somehow, but what would pay for the huge investments required for such an undertaking? It's a desert. There's all the dust, sand and rocks you could want, and a little water ice as well (but not too much) but that's it.
Forget the very real problems of terraforming Mars. What will really make sure Mars will never be colonized (barring a single, small scientific outpost perhaps, which is NOT colonisation) is the simple fact that there's no profit to be made on such a venture. The cost of lifting anything to Mars are so great that you would need to find something like Unobtanium on Mars to make it worth it.
The moon suffers from similar problems, although to a far lesser degree; it's closer and therefore cheaper to reach, and Helium-3 might (!) be the bonanza that could potentially make it worth it. But what profitable commodities does Mars have to offer to cover the costs of colonization, exploitation and transportation, AND make a profit on top of that? If there is one, I can't see it.
Biting the hand that feeds IT
