There are two things at work here. First is heat dissipation, which has already been mentioned. Those tiles are specifically designed to have INCREDIBLY slow heat propagation. This is, of course, to prevent the friction and compression heat from reaching the aircraft structure during reentry, but once the tiles are heated up, they are just as slow to cool off. There’s a famous publicity photo of a chunk of tile material that had been heated to incandescence, but someone (whom I hope got hazardous duty pay) is shown holding it by the corners with their bare fingers!

The second issue is volatiles. The APU’s in the tail end (which run the hydraulic pumps and other miscellaneous equipment) run off of hydrazine, which is both toxic and corrosive. They want to make sure that the H’zine and other volatiles have completely dissipated before approaching the vehicle.

- Jack

Remember, the tiles are like the coals in a firewalking fire – they may be hot, but they give out heat slowly. That way they don’t transfer heat to the Shuttle’s structure underneath.

http://www-pao.ksc.nasa.gov/kscpao/nasafact/pdf/LandingSS-2005.pdf

But some of the reasons it sits there so long is because checklists have to be gone through, switches have to be safed, etc. They can’t just land and jump out.