The main reason is that Trump was a one-issue candidate, and with the President's release of his long form birth certificate the number of birthers have fallen significantly (thank God). Additionally Trump's support amongst birthers has fallen also.
Beyond the overall support numbers, Trump is simply not like by Republicans. As he got more exposure, voters decided that they were not happy with a cussing, birther whose GOP credentials are at best suspect. Only 34% of GOP voters have a favorable opinion of Trump to 53% who view him in a negative light. A 53% negative is hard to overcome.
Keeping in mind that it is still much too early in the campaign to start following polls seriously, there does seem to be an interesting trend emerging. Romney and Huckabee may be starting Gingrich and Palin. Of course out of the four only Gingrich has declared (actually he as declared that he is going to declare tomorrow).
The survey points out that it is still anyone's game:
Now that (hopefully) Trump will be gone, all we need is for Ron Paul to go and the GOP concentrate on the REAL candidates.
We also looked at a fair number of alternate Republican universes in which various candidates don't actually end up making the race. With Trump out Romney leads Huckabee 21-20 with Gingrich at 15% and Palin at 14%. With Trump and Huckabee out Romney is tops at 24% to 20% for Gingrich, 17% for Palin, and 12% for Paul. With Trump and Palin out Huckabee has the advantage with 24% to Romney's 22%, and Gingrich's 20%. And with none of Trump, Huckabee, and Palin running Romney gets 28% to 26% for Gingrich 12% for Paul, and 11% for Bachmann.