Vince McMahon is keen to regain his formerly-firm stranglehold on the pro wrestling biz, and part of his re-monopolisation strategy includes stockpiling talent so others can't have it. This isn't a new phenomenon for WWE; back in the 1980s, the company ruthlessly pursued wrestlers from other organisations (like the NWA and other territorial groups, for example) and hoarded as many talented workers as possible.
They're doing the exact same thing today in 2020.
The problem is that there's just not enough space for everybody, not even when WWE has three hours of Raw, two of SmackDown, two of NXT, one of NXT UK and other peripheral programming like Main Event every single week. The writing team can't sustain the sheer weights of numbers on company books, and that's partly why some are struggling.
In fact, going a step further, they're totally wasted on WWE and might be better off elsewhere. Of course, that's easy to say when people have families to feed and are still making good money, but it must be creatively exhausting for these workers to see their efforts go unappreciated...