WWE NXT exists in somewhat of a paradoxical state. The in-ring product and storytelling is excellent and their Takeover specials regularly outclass any PPV that main roster can provide (Takeover: Brooklyn is one of the best major shows that WWE have run since Money in the Bank 2011).
Ironically, NXT has become the closest thing main roster WWE has had as competition since the Monday Night Wars of the late-90s, early-00s (TNA does not count). It has been hailed as the hardcore alternative to the more casual Monday Night Raw and SmackDown Live, as well as being what WWE's revival of ECW should have been.
However, that does not change the fact that NXT is WWE's developmental territory, where future stars hone their craft in preparation for Monday Night Raw and SmackDown.
Unfortunately, in its current form, NXT is failing as a place where performers learn the WWE-style, rather being an independent promotion under the WWE umbrella.
There are many reasons for this disconnect between what NXT should be and what it actually is, but these four stand out as to why NXT isn't doing its job.