Because Bush and the open-borders crowd really DOESN'T want to build it. It's like Chertoff said after the Senate bill died about enforcing the immigration laws, that we'll now start to see stories about families pulled apart, etc., resulting from a new emphasis on enforcement.
They hope that publicity from local opposition to the fence and sob stories in the press about "undocumented Americans" being sent back to Mexico will turn public opinion around and then they can get their amnesty bill passed, do some token work on a "virtual fence" and get back to building a superhighway for Mexican trucks into the heartland.
What Mamzelle just said, but yes, you added some detail to it. Thanks.