Admittedly, I didn't play more than 15-20 hours into the game before I lost interest, but XIII didn't even have stores or traditional towns. I mean, come on.
Everything was handled by those orb things, and not NPCs. I didn't find much there in the way of minigames or sidequests to break up the monotony of the fighting. While the fighting system was interesting in concept, I felt like I was commanding the fighting from the sidelines instead of having a more hands on approach, and that just wasn't as engaging for me.
I was left with no reason to explore, and nothing to do besides continuing forward with bland fight after bland fight. Since I didn't find the story engaging or adequately explained (far too many major story elements were left explained in the glossary of terms), I wasn't even motivated in those fights.
So piles of bland fights, no exploring of areas or cool towns to break up the traveling a little bit, a rather linear character ability/growth system, etc.
Honestly, if the characters, story, and combat system had felt more engaging I still might not have cared. However, since they didn't interest me much at all, it made all the other limited aspects of the first part of the game too boring to play. All I was left with that I liked was the very nice music, and amazing graphics. That's not even close to enough to make me care.
It's the first Final Fantasy I actually stopped playing because I actively disliked it. Others had enough redeeming qualities, but that one was a combination of things I actively disliked and things that were forgettable and boring. For me personally, that's why it's linearity stuck out so badly.
Final Fantasy 10 is not my favorite one, but it does offer a lot more to do in the opening part of the game to keep me interested, combined with a more traditional combat system that I enjoy better. I also found the characters and story more interesting as well. That certainly helps.