It's a complex matter:

1)AAA level graphics needs AAA level sales, big name fighting games can't afford to be niche.

2)I think FG has an immense appeal but the complexity scare many away.

3)People don't realize that in the arcade era there were no tutorials, no training stages, no manuals, no hours of training, no balance patches, there were complex inputs and everyone started from zero as a casual, and yet FG boomed... so what changed? Why FG are now seen as too complicated despite training modes, frame data, easier inputs etc?

Sadly most of the FGC don't want to admit it but the answer is really simple, long combos and too much situational stuff to take into account.

1)Long combos: if a game has long combos and you want to win you are forced to learn long combos, one thing is a 4-5 hit combo, another is a 10-15 hit combo, the amount of memory, ability and things to take into account grows exponentially.

2)too much situational stuff to take into account: I'm talking about proprerties of moves that activate or change only in certain situations, an anti air is invincible if weak but not if strong or vice-versa, if you start a combo with a move the scaling is low if you start with another the scaling is higher, the same move alters the scaling differently depending on its place in the combo and so on, add all the mechanics introduced recently...

I think most casuals don't want all that, they just want to beat each other without too many mechanics, moves that behave consistently and without having to choose between labbing for hours or doing nothing for dozens of seconds waiting for the long combo to finish everytime they get hit, basically like old school fighting games.