MOBAs suffer from the problem that, unlike other genres, you specifically rely on your teammates to be able to win. In FPSes you can, in theory, win every game by yourself, regardless of what class you choose. In MOBAs, no. Every role needs to work with the team in order to shine. The problem is that everyone in pubs wants to be carries or gankers in order to be able to jack off their big K/D-penis, and that just doesn't bloody work. Teams need support players, but that's really no fun when 50% of the time people will rage at you for getting a shit K/D-rating(even though you sacrificed yourself five times to save the life of your retarded carry), 45% of the time people don't give a damn about the fact that your warding/pulling/set-ups are winning the game, and only 5% of the time will they applaud your skills at keeping the team afloat. The problem then, however, is the fact that being a good support doesn't carry over games. Nobody gives a shit about your ward score when you have < 1 KDR -.-
Or to put it another way: MOBAs are at their best when you have an organized team rather than playing pubs. Nobody's gonna rage at you, and you don't have to interact with shitty, raging 14 year olds who builds Scout with Codex and proceeds to complain that their team is shit.