In a way, i guess that you are right.
The game play for Mortal Kombat is not as deep, for, say SF4 and SF5.
However that said, in the Fighting Games e-sports scene, it appears that only the Street Fighter series has gained any serious traction as a Competitive Spectator e-sport because of its longevity, over 25 years. Its deep game play, its huge cast of unique characters, its lore. (Sounds like a game we know? 
It was also the progenitor of “true” fighting games in the history of arcade games when SF2 was launched. So people would always return to the origin. However, that said, while it has always been big in Japan, it was really the U.S. which brought it back into prominence as only you Yanks can 
. There are a few documentaries you can watch. This is a good one to start:
For a good introduction of fighting e-sports, this is a good article.
Once you start on Fighting games, my friend…well… it is a life long love.
If you like watching anime, there is a really good one which brings back my childhood memories.
High Score Girl: