In MMA you only really need to be okay at a bunch of different things with one "specialty" which will allow you to differentiate yourself; this is what has allowed for mediocre boxers and MT guys like Silva and Connor to effectively dominate the standup game and guys like GSP to grapplefuck their competition for years on end. It's a sport of inclusion wherein the required level of mastery for all its constituent components is significantly lower than in a specialized sport like boxing and any weaknesses in your game can be mitigated by crutching heavily on your strengths. What I mean by this is that you don't need to be a good wrestler, a good striker and a good submission artist to be successful in MMA, in fact you could be a terrible striker, a terrible wrestler and a good grappler and be UFC champion (I'm looking at you Honda). In boxing you have to be at the bare minimum good at all aspects of the sport with at least one area where you excel to even be considered a contender and this is due to the depth entailed by a sport of reduction. In MMA you could conceivably achieve a competitive familiarity with the ground as well as the striking game even if you are not especially talented or athletically gifted and oftentimes you can get by without any serious, nuanced knowledge of either aspect which is required to simply be competitive in boxing or really any specialized martial art.
Another way to think about is crossfit versus it's constituent components; the average games competitor does not possess strength or endurance beyond what's achievable by a regular person with a few years of training. And while it's impressive that they can run a few miles and also do some ugly cleans afterwards I guarantee you that maybe one out of every 100 "elite" level crossfitters is even genetically capable of achieving the same level of mastery in marathoning/triathloning, or even approaching world record numbers in weightlifting or powerlifting.
TL;DR: MMA sets the bar lower for each of it's constituent elements which means that there are fewer barriers to entry into the sport and the skill ceiling is significantly lower because all these elements lack depth.
