Because the game is comfortably in a spot where it continues to print money without needing substantial updates. Put bluntly, there's zero incentive from a business prospect to invest significant resources into a project that's more or less self sufficient with the bread crumbs on often.

Think of it like this. They could invest an additional 20-30M into FFXIV but would that bring in a wealth of new players? Probably not. On the flipside, putting that money towards another game could very easily pay it back and then some. Unfortunately, SE's attempts at new IPs or anything not Final Fantasy or Dragon Quest related have been woefully inept. But that isn't a flaw on the concept itself just their poor decision making.