Well the thing is, these are older systems we're talking about.
The people that buy these games already own an N64 or a PSX. They aren't going to go.. "hmm, I want an Episode I game, doH! It's only for that newfangled N sixty, whatever, I guess I'll have to buy that instead of a PSX, just for this one game!"
These systems have been out for years. Most people that were going to buy one have done so already. This is just another game for them to buy. Some people have multiple systems, this is a way to get them to buy both games.