Tidus is known enough to be in SC 2. FFX, sadly, sold quite a lot, on its secret release date. He's just too much of a pansy to step up to any of the SC fighters. I'm also pretty sure a good portion of SC fans would be pissed at seeing someone like Tidus in SC 2. It's about toughness, guts, and whatnot. The characters from Soul Calibur have quotes that would make Tidus pee his pants, throw them out, then pee on them again. I can see Tidus' entrance and victory quotes now:
Entrance: Aaaaaron! My dad hates me! WAH.
Victory: HA. HA. HA. HA. HA.