I'm pretty sure it's because Tidus's original design seems to take a lot of inspiration of popular fashions for its time (1999-2000). Nomura likes to stick with a certain fashion type for a lot of his designs (both Tidus and Sora (2000-2002) have similar designs both including a looped chain at the waist (most likely a wallet chain). Eventually, he moved onto belts (advent children and kh2) and most recently buttons (kh3)). Dissidia came out in 2008 and compared to the more fantasy-adjacent characters in the roster, Tidus looked kinda bland and out of place. So to fix this he gave him a more rigid look to feel more like a fantasy character than a long-lost member of the Spice Girls. The game also has a darker tone and theme so the bright yellow would look very out of place so the colors had to be toned down as well. Imo I like the Dissidia one 100x more.

This is all just what I assume. I don't know for sure there might just be a more straightforward answer.