2014 - All I'm going to say.
My generation (I'm 21) grew up and seem to be mostly filled with a bunch of shits. Let's not talk about the up and coming!
Nah that is one of the reasons, but I think there's some others also.
Now days you can PuG and scream and bitch all you like without a consequence. Back when before LFD everything all pugs/groups were done with people all on the same server. The community (From my experience) made the toxic players known and most of the server knew who to avoid and who was someone good to group with. There was a bigger consequence pre LFD which lead to toxic players ending up being screwed if they had a bad name. I'm sure this wasn't 100% the case for everyone, but at least from my experience.