A number of factors drove the decision - financial, political, personal - but chief among them was the desire to worship without being on display. Obama was reportedly taken aback by the circus stirred up by his visit to 19th Street Baptist in January. Lines started forming three hours before the morning service, and many longtime members were literally left out in the cold as the church filled with outsiders eager to see the new President. Even at St. John's, which is so accustomed to presidential visitors that it is known as the "Church of the Presidents," worshippers couldn't help themselves from snapping photos of Obama on their camera phones as they walked down the aisle past him to take communion.
White House aides say that security measures required by the Secret Service have become stricter since 9/11 and would cause significant delays for parishioners - and at significant cost to taxpayers - on Sunday mornings. Given Obama's popularity within the African-American community, the President also worried that if he chose a local black congregation, church members would find themselves competing with sightseers for space in the pews.
Update: According to Press Secretary Gibbs, there is not much truth to the fact that Obama has chosen his church. He is still in process of deciding, but, it is true, that he does not like to disrupt worship services.