User blog comment:Kogath/Monster Hunter XX Discussion/@comment-33581177-20170208202857/@comment-27120688-20170211045530

Subs and deviants aren't the same though. True, their reasoning for not putting them in MHX were so that they could put back more unique monsters while not confusing people between subs and deviants. But subspecies are that, a subspecies of said monster, while deviants are crazy versions of the same monster species. They fill two roles in the series. Deviants are actually closer to Variants (and are even called Named Variants in Japanese).

Plus, Deviant equipment does not serve as a replacement for U and Z type equipment in the series. Deviant equipment give you a very unique armor skill, which can only be activated after you level up your armors, AND have no decoration slots. U/Z armors offer more variety and sometimes better choices than your standard S/X armor sets. Both have their positives and negatives, and both should be in the game imo.