Rather stay in the private sector & make money.

From NRO:

After considerable deliberation and reflection, I have decided not to pursue the office of the Presidency. This decision does not come easily or without regret; especially when my potential candidacy continues to be validated by ranking at the top of the Republican contenders in polls across the country. I maintain the strong conviction that if I were to run, I would be able to win the primary and ultimately, the general election. I have spent the past several months unofficially campaigning and recognize that running for public office cannot be done half heartedly. Ultimately, however, business is my greatest passion and I am not ready to leave the private sector.

More at NRO.

Also, <A HREF="http://insidetv.ew.com/2011/05/15/nbc-if-donald-trump-runs-for-office-well-replace-him/">NBC threatened</A> to replace Trump on The Apprentice if he ran.