Cuz they are/were generally older and past their prime when they fight/fought in the UFC. Most of them were already fighting when the UFC was sill a small organization. The mileage Pride fighters had when they arrived in the UFC didn't help.
Great Pride fighters would probably be as dominant in the UFC today as they were back in the Pride days if they were still ~27-28yo today and had the same training methods young fighters have today.
The sport evolves, the training methods evolve, they way to do things evolve... but the talent/heart/will to win are not something you can learn. You have it or not, and I think guys like Fedor, Hunt, Coleman, Crocop had a lot of talent/heart/determination.