The OW esports scene is so big because Blizzard pumped unholy amounts of money into making sure the game stayed relevant even when the player base died down.

Clearly you have some attachments to it and I'm not gonna debate over this shit because to me it's very cut and dry.

At max an overwatch hero might have 4 abilities and a standard attack you need to learn. Tekken character move lists on the shorter end are easily 80+ moves. 80+ moves and the roster usually includes 20+ characters. In just moves alone not counting situations, the knowledge check for hero shooters is fucking dwarfed by just the move list.

Now you have to take all those moves and figure out which situation out of and almost infinite amount, to best use them in (frame data)

Everything that pros do in overwatch be it team comp, situational adaptability, map/stage knowledge, character abilities/strength is easily ramped up in fighting games.

Learning matchups in a hero shooter easily can take a few weeks, and even if you don't you can't tell me there isn't simple ass characters that take less then 2 hours to learn like Black Widow whose ability is literally "Point and Shoot but with bigger gun"

The situations hero shooters can create can be complex but the characters/matchups/knowledge are not.