I don't know what the situation is now that Blizzard is no longer really Blizzard, but back in the early years of WoW this was - in large part - because almost every other MMO that launched just plain sucked.

Blizzard was to the gaming industry what people often believe Apple is to the hardware industry. They were the only ones that invested in polished UI, coherent UX, etc. and it was so incredibly noticeable.

I'm sure a lot of this was pressure to get something out quickly to make a "WoW killer", which is what gaming media branded basically every MMO that launched after WoW.