Because he's been too successful, I guess. The impact of Obama's tax proposals:
It was fortunate for Tiger that his most-recent U.S. Open win occurred in 2008. Under twin tax proposals from Obama to 1) remove the "cap" from Social Security taxes for individuals earning over $250,000, a plateau Tiger has long since surpassed in 2008, and 2) eliminate the "Bush" tax cuts, thereby raising the top marginal federal income tax rate to 39.6%, Tiger's taxes on his winner's check would have increased to approximately $776,000, a boost of almost $190,000. Instead of Tiger keeping 57% of his earnings and the government taking 43%, under the twin Obama tax proposals, Tiger's federal and California taxes would have amounted to 57% of his winnings, leaving Tiger with just 43%
(Hat tip: TaxProf)