I was really disppointed to see Ject in Dissidia instead of Seymour. Jecht was never a real villain! But it goes to show that maybe, Seymour didn't make enough of an impact on the fans.

Personally, I think it's because he didn't have any real connection with the game's protagonist, Tidus. Look at the other main villains of the series: Kefka, controlling Terra. Sephiroth, used to work with Cloud, both injected with Mako etc. Kuja is Zidane's "brother". Seymour...... has no real connection with Tidus at all.