Merger. Whenever a company acquires another the first thing they want to do is exploit the portfolio. When Square became Square Enix, that's when X-2 happened, the first sequel, and from there it's been FFVII compilation, Ivalice Alliance, DS remakes, PSP ports, Kingdom Hearts handheld spinoffs, Dissidia and more and more Final Fantasy XIII.

It's natural for them to want to get more out of the years of work they put into making XIII, but you have to question why make it in the same universe.