Why do MMORPG communities always suck?

I have played EVERY single popular MMO known to man (both free and pay-to-play) and not one of them had a decent community. I remember playing this one small MMO which had only had a few hundred players and it was alright for a while, but once the game was advertised more, then a lot of shitty people came and ruined it. Why?

I loveddd the community and economy of FFXI but the rest of the game was crap, and shadowbane was fucking AMAZING all around but the servers crashed like every ten minutes.

and yes
>>20
everyone on WoW is retarded