The ps2 was the bestselling console of ALLTIME. No console before it or after sold as much as it, which I believe is at over 140 million. Because so many people have it, and the fact the ps3 came out a year later than the wii and xbox 360, it is going to last longer than the previous gen consoles.
xbox 360 is a good system so they moved onto making for that insted of releases for the xbox. PS3 has a poor line-up to devote to it so they continue to make games for the wonderful old PS2 which was the best consol of that generation.
Microsoft has no interest in developing for a console that they upgraded from almost three years ago. The PS3 is a lot newer, so the PS2 still has some way to go yet.