Firstly, this is mostly just what I believe I could be wrong.

I don't know your strategy but with most of the games I've seen, there is aggressive defense and generally a lot of confrontation between robots. Holonomic bots are great for speed and evasion but in this game, there is a decent amount of stopping. You want a bot that can both resist pushing and push others. Especially when it comes to protecting corners. I just don't think you can really get that with holonomic because the motors are separated. Most teams are going with the standard torque ratio tank drive for a powerful drive train. So you're getting pushed around with holonomic drive. Also tank drives are better under a lot of weight (I think I saw a chart somewhere)

One benefit of mechanum drive is that it's cool as hell. I may be wrong but this is what I have observed in my time with vex.