#2 To keep NXT strong
One thing that one must always remember about wrestling is that it's never a destination, but a journey. So when one chapter ends, another begins immediately after. Sometimes on the night after!
So, let's take a look at an act like The Undisputed ERA which a lot of people are claiming should be called up to the main roster right away because they seem ready to go and create an instant splash. If they do get called up, who are the potential antagonists that Johnny Gargano can face over the next round of tapings? A Champion is only as strong as the challengers he faces during his reign.
Check out which wrestler EC3 finds cute RIGHT HERE
So one must remember that while the ultimate destination may be the main roster, for everyone who comes to NXT, the black and yellow brand also needs to be kept very strong. Which is why, probably, no call-ups happened after WrestleMania.