honestly as an outsider, in the nicest way possible, i think it's 'cause nrs games are considered a joke by most of the competitive fgc. tournament numbers for NRS games are oddly consistent, so i suspect it's mostly a particular group of people who somehow enjoy playing NRS games competitively. i think most people who play NRS games competitively were simply drawn towards the game itself, and then they enjoyed the game enough that they wanted to compete, whereas people who play xrd, sf, skullgirls or KI were drawn towards the genre itself.
i say this because mkx is fraudulent on a lot of levels, and many fundamental 2d fighter skills are non-existent for this game. so, people who are serious about fighters, but aren't NRS fanboys already, will generally avoid this game for competitive purposes.
then include how many balance adjustments there are, and how incompetent NRS are when it comes to actually balancing properly, and you end up with something that's considered half a joke by most of the community.