President Bush traveled to an Arizona town on the Mexican border today to try to build new momentum for an immigration bill

Later in the article:

The Senate passed a bill last year that would put most of the country’s illegal aliens on a path to citizenship, but the idea stalled in the House, where there was more opposition from conservatives. Prospects for a bill emerging from the full Congress seem even less favorable now, as Democrats and Republicans alike have taken note of anti-immigration sentiment among their constituents.

Moreover, politicians who are running, or thinking of running, for president in 2008 have encountered deep resentment toward illegal immigrants in Iowa, where meatpacking plants rely heavily on immigrant workers. Iowa’s early caucus contests are regarded as crucial to any candidate’s chances.

Now, when the administration is embroiled in an unpopular and unnecessary war, dealing with major scandals and security fiascos I don't see why pushing an unpopular immigration policy is on Bush's agenda.

The best reasonable explaination I can think of is someone in Latin America has some really good dirt on W.