Why so many FInal Fantasy XIII??

Depends. It is a tactical combat simulator that puts every ounce of gameplay into the combat mechanics, and loses many other JRPG tropes (like city hubs for shops). Also, the story is confusing until you get to the end and read all the data logs. (The story is actually good and the lore is astounding, but the dramaturgy is just god awful, so it has troubles actually telling it. Big troubles.)

It is as linear as all other FFs, but this time they literally show you the corridor and cut out the world maps.
So, in a sense, yes, it does give you the feeling of linearity. Only one open world chapter is there, but that also has all the side-quests (similar to the hunting missions in FFXII), so it can make up more than 50% of the game time. It also has its fair share of exploration, mostly in the form of chocobo riding treasure hunt.
But the real strength is the combat. It is often accused of being "Press X to win", but in reality it is the most complex combat system in an FF, rivalling Tactics and somewhat surpassing X-2. (And also the first FF where disabling spells such as slow or stop not only actually work on enemies, but the role that does these spells is one of the most useful ones.)

I also have to add that the sequel, XIII-2 added back most of the missing standard JRPG stuff (city hubs, world map, 99 levels instead of 5, and even a pokémon mechanic). The third one is more of an open world JRPG/hack & slash hybrid though.

I'd say it is a game probably not for everyone, but for me, it is my second favourite Final Fantasy after VI.
And when I get a new video card in May (or my next rig in 2017), that 250 hours will hopefully multiply. :)

FF is a good series. Sure, not as imaginative as its title or some of its fans make it out to be, but it really is a good series of JRPGs. And FFVII is a good game too, but nowhere near the masterpiece many of its fans like to sell it as. Just most people met it first due to the insane Western marketing Square made befire the release, and as a general rule, it can be said that the FF almost everyone ranks as their "best" is the one they played for the first time.