As others have stated, it's the monotony. It's the same rooms and passageways copy/pasted over and over and over again. Other areas that preceded it were every bit as linear, but they at least had some variety in the environments.
And what made it worse was the placement of the Cid battle. Being the most intense and difficult boss battle up to that point, you naturally expect it to be placed at the end of the chapter so that you can get a bit of a breather afterward. But instead, they inexplicably decided to put it in the middle of the chapter, making the endless tedium that follows even more excruciating.
Seriously, fuck Chapter 10. I don't think I've ever hated a game level so much (which is quite an accomplishment on Squeenix's part).