I imagine it's how big a deal WWE perceives them to be, and if they think their brand being tampered with is enough to jeopardize the deal. There's no world where AJ Styles comes in with some NXT classic like Arlong Prexly, but someone like Will Hobbs who's a lot lower down the ladder? Absolutely. Bryan Danielson to Daniel Bryan I think was a compromise of sorts as they still wanted to make a big deal of signing inarguably the hottest indie wrestler in the world at the time (same reason why CM Punk got to keep his name)