The Tony Hawk "franchise" was never a Playstation exclusive. Just look at the following:
TONY HAWK\'S PRO SKATER
- playstation, nintendo 64, pc, dreamcast, game boy
TONY HAWK\'S PRO SKATER 2
- playstation, dreamcast, game boy, pc, nintendo 64 (soon), x-box (soon)
TONY HAWK\'S PRO SKATER 3
- playstation(2), x-box, game boy(advance), pc
As you can see, Tony Hawk has been on every system released, so it was highly unlikely that Activision would have made it just for Sony.