As someone who is more in the Nintendo line, I cannot say for sure, but I'd guess online console interaction was young. Players were more willing to cooperate and be friendlier then they are now, some 8ish years after the XBox 360 release.
Competitive nature. That is all I can really say. People want to win. They allow that urge to win transform into something terrible and this is the result. Whenever I play online PvE content, say Monster Hunter 3 Ultimate, I have never been exposed to anything negative at all. Destiny Raids? None.
PvP is much more competitive so it brings out the worst more.