Tekken 6 is old and isn't all that great of a Tekken game, should have gone with Tag 2. Tekken 4 is considered the worse game in the series.
Not because it isn't fun, but, because it's broken. Tekken 5 was a lot more balanced, and brought the gameplay closer to Tekken 3. Tekken 6 was hit with criticism over rage mode and bound moves, and, when combined with floor breaks, some stages gave characters touch of death combos.
Now, would it have sold better as an exclusive? No.
Tekken 6 had to become an exclusive to get the sales it ended up with. It wasn't good enough of a game to make you want a PS3. Games have been tarnishing. Tekken 6 might be considered better than Tekken 5: Dark Resurrection by some, but, it doesn't make enough of an improvement over it's predecessor.
As quality in games fall,or the quality of sequels don't improve enough to make a big difference, then they'll have to be multiplats to maintain sales.
Tekken 1, 2 had a few improvements. Tekken 2-3 had tons of improvement. Tekken 4-5 fixed the problems of 4, though, I think the uneven flooring should have stayed for some stages. Then, Tekken 6 screws everything up again by adding stuff people don't want and then the console version is noticeably worse than the arcade experience.