A Great Question
First, the contention by some on the left that this is the Democrats standing up to George Bush is a crock. Were the Democrats standing up to Bush the bill that was tabled by Pelosi would never have made it to her. Instead, Senate Democrats and Republican's worked together on a bill that would allow intel agencies to continue their work, telecom's to be protected for cooperating, and require FISA review of all surveillence within 72 hours of starting such "eavesdropping".
So, why is Pelosi holding onto it, instead of letting the House vote? That's easy, and annoying, she's placating the Daily Kos/MoveOn.org wing of the Democratic Party. The far left of the party is very very unhappy. Democrats have controlled Congress for over a year, and we're not only still in Iraq, we're doing a better job than they can stomach. No one has introduced articles of impeachment for Bush, and none of their pet projects have come to fruition.
Two years ago the MoveOn guys and Kossites decided to attack any Democrat who didn't vow to do their bidding. While they were wildly unsuccessful at getting folks they wanted into office, they cost the party a lot of money defending incumbents who were attacked, and distracted a lot of candidates from other messages.
Now, election season is starting to heat up, and Pelosi doesn't want to deal with those folks again, so instead of giving them something to complain about, the passing of a bipartisan piece of legislation, she's decided to calm them down, at our expense. At least that's my theory.