Because as cool as Versus sounds, I'd rather the next mainline title not be a renamed action-RPG spinoff announced in 2006 with a new subtitle. Seems like the easy way out for a company that really needs to apply itself. I'd like a return to form, and I guess you could argue that the subtitle and if/when that game actually comes out don't really matter, but I think fundamentally it would just be good for the brand it Versus remained its own thing and the next game was actually intended to be the next Final Fantasy from the beginning.
And yeah, marketing it as XV might cause them to artificially delay the next actual title a bit longer, though I guess the XIII/XIV release gap is an argument against that.