Well, I looked at Japanese trailer, it was showing scenes from Metal Gear Solid 3 and Peace Walker. I got to say, the scenes of Peace Walker, cut scenes and game play alike, are like from a PS1 game compare to scenes of Metal Gear Solid 3. Therefore, there are only two reasons I can think of as why they did not release it for PS2. The first and the obvious one is that PS2 is reaching the end of its life and games are barely being release for it anymore. And the second one is the online/multi-player capabilities on PS2 is virtually non-existent.
So if they do somehow remake it for PS3, hopefully they use Metal Gear Solid 4 engine or at least Metal Gear Solid 3 engine.