There are simply too many major issues he has to deal with that are also extremely important. He has to work on health care, cap and trade, and the recession. Each of these require so much attention by the US that other issues such as immigration reform and gay rights have to be dealt with at another time.

The issue of immigration is very sensitive to many politicians, and american citizen, who fear of losing jobs to million of immigrant who could become american citizen, specially lot of immigrant are also skill and educated,
I think immigration issue is best dealt with when economy is more strong and stable able to handle the million of additional citizen.

The short answer is political calculation in Congress.

Immigration is number 3 in the list after health care reform, and banking reform. The first 2 legislations would take up a lot of political cachet throughout the rest of this year and first half of next year, just to get pass congress. After that it's a congressional election year, when typically, very little gets done. Also, many politicians would also be afraid to touch a sensitive issue like immigration during an election year especially in the middle of bad economy when unemployment is above 9%. Which means, realistically, immigration reform could be pushed back even further - after 2010.

Republicans may pick up some seats in congress next year by feeding on the popular conservative anger towards the democrats. That would mean less votes to pass a Comprehensive Immigration Reform (known as CIR in Capitol Hill) after 2010. But that's not necessarily a bad thing for the democrats. If large blocks of Hispanic voters stay dissatisfied with the republicans because they keep blocking CIR (as they did in 2006), then that translates into a larger democratic constituency. You could argue that passing CIR is politically harmful for both the republicans and the democrats.