Its because NXT doesn't give a fukk about developing new wrestlers; its WWE wanting to crush the smaller promotions like ROH, GWF and whatever other indy isn't lowkey on the payroll like Evolve is.
FCW/NXT WAS about developing talent before it became a touring brand. For every indie star they had like Seth/Dean/Cesaro (who all needed development in various areas), you'd have 5 Big Es or Romans or Bos who were fresh and needed full developmental experience. Once the Network came along and exposed NXT to a bigger audience than just local small Florida cities and Sunshine network, it became more imperative to hire more "names" as the brand was growing hot enough to tour. But they were still developing talent. Sami certainly wasn't used to working maskless and cutting promos on TV. Tons of guys would wrestle as themselves for a few weeks then come back trying out new gimmicks. Balor and Itami weren't used to working American television matches.
So once they got hot enough to tour, obviously they weren't going to be competing mainline WWE, the direct competition became bigger indies like ROH. And then once you have to worry about selling tickets all around the country, you need to get bigger names from different places instead of relying on PC talent that haven't been on TV in months. And then they got bigger and bigger so the talent had to get bigger and bigger, and now they're full blown "hire any big name that's available and throw them on TV immediately so they can go on tour for the next trip" mode. NXT isn't going to sell out Barclay's with the various Indian and Chinese talent they've recently seen who only work in front of 120 fans in Florida. Does anyone think they would have sold out BC in 2015 if Joe, Balor, Liger, and KO weren't on the card?
Although, NXT has effectively been two promotions since the touring started, with the local Florida loop being 90% PC people that aren't even ready for TV development yet, then the main touring/TV brand.