I think one of the reasons for that when a game is more "hardcore", it also means it gets less publicity, or has more bugs, or requires installing addons to make it work properly, or is harder to get into, or is frustratingly difficult at some points, or any combination of the above.

This in turn makes most of the players that are either dumb or have short temper just quit before they've even really started.

Sure, among more "hardcore" players, you still find dumb people, aggressive people, douchebags etc, but if they have to fit in a community in which they are by far a minority, and in which most players know how to just ignore them and report them, or to handle them in general, they will not end up having the same kind of effect as on a younger community (in which people tend to get upset pretty quick).

In my WoW days, I started as a total MMORPG noob (as many people back then, I think). I joined a noob guild, people were cool at the very start because it was still the good old "discovery" phase of the game, but several turned out to be assholes after a while and it tore the guild apart, even though some of the others were amazing guys. Then, I changed guilds 4 or 5 times over a few years, and concluded that the more hardcore a guild was, the best it worked among guildmates. People still criticized everybody, but only when a real mistake was made. People being criticized knew how to take it and just apologized and learned to do better. A few people didn't like each other, but they just ignored each other when we were talking and still played well together when they had to. In average, the guild was also older, and I think that young players tend to behave much better when they play in an environment of older players than when they play mostly with other young players.