Short answer: 1) In an attempt to impress its Tea Party base. 2) Because two of its front-runners are prescribe to a religious ideology wherein it's unfathomable that man could foul the big G's creation -- this is also why Rick Perry and Michele Bachmann literally believe climate science is a hoax. 3) The money's pouring in from industry interests. This decent clip hits on 1 & 3, but it's important not to discount #2. I've been guilty of not acknowledging roundly enough the deep ideological roots in the recent rise in anti-science and anti-regulation sentiment, but be assured it's there. Check out Ryan Lizza's New Yorker profile on Michele Bachmann to get a sense of how her deep connections with an arcane sect of Christianity called Dominionism, whose primary belief is that man has (you guessed it) dominion over everything else, informs her worldview.
Perry has similar ties himself. And that's not necessarily a bad thing outright -- it's just that these particular ideologies serve to inculcate in believers the sense that the environment is either invincible or ours for the plundering, and that it's ridiculous to think we could be doing anything as severe as altering the climate with human activity. This makes it eminently difficult to have a reasonable policy discussion on scientific matters like climate change, as we've seen.
But the Tea Party and money are big drivers too; I'd argue corporate money is the biggest, and Ed Markey makes a pretty good, if sound bye-laden case for why.