Well, that too. But it's the fundamental map design. On the one hand, Turbine is arguably the most well balanced CTF map in TF2, mostly because it's unnecessary to fight past the enemy spawn room to get to the intelligence. On the other hand, there's absolutely no cover anywhere except in the middle room. It's a giant 512 unit wide hallway the entire way around. It reeks of poor map design, and I'm convinced that the only reason the left hallway bends the way it does is for map optimization reasons. Also, I don't think the visuals also aren't up to par with any other map in the game, but that's secondary, not to mention subjective.
tl;dr boring map layout