Obviously there’s a correlation between the number of heroes in a role and the number of players out there interested in playing that role.
If that was true we’d expect more tanks than healers, but there are something like twice as many healers as tanks, judging by the queue times.
People come to shooter games to kill stuff, and obviously healers and tanks don’t accommodate that desire nearly as well.
It simply doesn’t make sense for a hero like Moira to be in almost every game if she didn’t offer something other supports don’t - kill potential, the ability to not be reliant on her team to succeed. This has 0 to do with the number of heroes in a class and everything to do with what the heroes can achieve in a game.