Fighting games take longer to produce, and have less market power. There are 5 fighting games, none of which can be said is inferior.
MvC3, SF, DoA, MK/INJ, and KoF all have seperate followings, and are all dynamically different. Their difference prevents all but a select few from being in shared pools.
Fighting games can't just be picked up and learned in a day, usually. The technical side turns away a lot of people, and the stigma against new players is there.
Mobas take less time to produce and all of them fit a genre with minor changes, so there's no learning curve outside of new mechanics. A skillshot is a skillshot, changing the game doesn't change what it is, just what it does.
Likewise, MoBAs and some shooters have huge market power. Followings also tend to be linked somewhat, where there are some loyalists but the pools draw from eachother.
MoBAs can be picked up for free in most cases, and give the player more benefit because of the team aspect. Bad players can get through games and learn slowly, good players can drag along bad teammates.
MoBAs also have more customization, and thus makes the player more responsible for their performance. Their choices in items, their skill in last hitting, and their ability to outplay is more important than in fighters.
Taking that into consideration, players are more likely to keep trying when they fail since they have room to change their build or decisions without having to per se dedicate 100 hours to it.
If they fail early, they can make up for it later. They can get ahead early and fall behind as well. In a fighter, you either win the round or lose it. If someone frauds you out, you don't get another 30 minutes to learn and go again.
Thats my idea on it at least. Fighting games are telling a fish to climb a tree versus a monkey. Sure the fish might do it, or even get better than the monkey, but otherwise the fish is like "why would I try to climb this tree? There is no point."
If the fish doesn't travel, the best they can have is online spammers and the occasional good player.
Meanwhile on Mobas, they can get up to rank 1 in tree climbing, or be a diamond tree climber, and it actually means something. The skill cap is automatically raised to suit them. They grow, and even if they can't travel, they aren't still stuck fighting bronze 5 or potato MMR people. Fighting games have tried this with skill ranking, but its too easy to climb skill and then you have trash in the higher rankings. Players see this and think "this is all the game has to offer". Meanwhile if you play a MOBA? You can't be terrible and still rank up, because eventually you'll hit that wall where you can no longer climb because those who stand before you are much better. Players who see you, beat you, pass you, and you fall down to where you belong.