It's an MMO. Group content should be the majority of content, not a few optional quests. Designing an MMO to be 95% solo game makes no sense at all. It would seem to be pretty obvious to me that making a multiplayer game actually mutiiplayer for the majority of the play experience would be a no-brainer.
Some people can only play 30 minutes at a time, and it can often take longer to get a group than it takes to run an instance. Others have to deal with unavoidable interruptions (i.e., kids) when they play, so committing to a group run is either impossible or irresponsible, when you might have to go /afk or log off for a while halfway through.
There is also a high correlation between people being in the situation of having little time or lots of interruptions, and those same people having more money to spend on the game (in other words, being the ones who dump a lot of cash on stuff like cash shop loot). Making a game that doesn't offer a lot of playable content for those people doesn't make an awful lot of sense from a developer's perspective, assuming that the developer is in this business to make some money.
Having lots of content that you have to group for is good, because requiring a group means it can give good rewards, for the increased time and effort put in. Making a game that requires a group for everything, all the time, is a great way to have a really tiny player base (and a really crappy looking quarterly report).