I love FF13 and I try not to bother too much with arguing about it.
Anyway the fact remains that it was—relatively speaking—a critical and commercial success, and like it or not, Square Enix's modus operandi with Final Fantasy is (and has been for a while now, so people really shouldn't be surprised) to treat each numbered Final Fantasy game as a base for a franchise. They did it with VII (Before Crisis, Crisis Core, Dirge of Cerberus), X (X-2), XII (Revenant Wings), and even IV (The After Years). XIII was conceived as a franchise from the start; they even gave it a fancy name: Fabula Nova Crystallis. Seeing as how the main game was successful (i.e. made them money) and they know what to do to make a better one, it's kind of a no-brainer to make a direct sequel.