Because other than the same sing-a-long entrance, they bring nothing to the table on the main roster. They're both pretty bad in-ring. Cass is as generic as they come: both in looks, wrestling & on the microphone. Enzo would be a better manager/mouthpiece and, like you said, they never won the NXT tag titles. They had more they could have done with them in NXT. And NXT could use them.
Pretty much. They're over because of the entrance routine and Enzo on the mic. Once that wraps up they really can't do much at this point to get themselves or any opponents over, although they did a nice job with Cass in the title match after Balor went down, but you can't exactly use those gimmicks every night with him. Could develop them a lot more in NXT right now.