Because despite being linear, it had good pacing, a great story, a great battle system, likable characters, and towns and mini-games to enjoy along the way. Final Fantasy XIII's story was mediocre and underdeveloped, the world was lifeless, the art direction was dull, and there were no breaks from the endless battles.
We must not confuse dissent with disloyalty. Dissent is the highest form of patriotism.
FFX had plenty of content to mask it's linearity. Even at the end of the game, there was tons of stuff to do.
FFXIII had... Adamantium farming.
This. FFXIII really took the linearity further with things like not allowing you to backtrack. Also the setting of FFX is better developed and felt more like an actual world. I also found the battle system and soundtrack to be a lot better in FFX and also liked the story and characters better as well. There are a lot of reason why I like FFX and dislike FFXIII but I think when it comes down to it the simplest explanation I can give is that even though FFX didn't have a world map like previous games everything else about it still felt like a FF game whereas FFXIII didn't feel like one. Of course FFXIII failing in other departments as an rpg didn't help either.