It's simple, some people build their ego because they think playing on PvP servers makes them hardcore.
The rest think the same as Undertanker here.
They somehow believe a game designed entirely around PvE content with frankly indefensibly bad PvP gameplay is meant to revolve around PvP.
I personally believe that PvP has never been good in WoW, as the game isn't build around it in the slightest (that part is fact not opinion), so I won't waste time on it.