Probably partly because the focus of action oriented combat makes it harder to fight when there are multiple people running around. But mostly though I think it is due to making mmos more solo friendly, which forces them to give all characters abilities that would enable them to cc,tank,dps to some extent. Giving all players those abilities changes how you design group content too. When everyone can do everything, you have certain specializations that might give you more hp or magic abilities and that defines you into a role, but most still carry around rudimentary skills to make them viable in any and all solo play. For instance CC abilities to handle adds, where the obvious example would be chain stunning. This would be very easy to do in a dungeon unless they add resists where once stunned, resists were raised. Stun then having to be a normalized term applied to the dozen different CC abilities the various classes have. Four mobs in the room, no problem cc three of them. Four people in group, so even have one to spare. Six people in the group would then trivialize the encounter.
So I think the sad and simple answer is that it is just easier to lower the number of people in a group compared to balancing it to the massive increase in power all players have alone now compared to a game like Everquest. I had no CC as a mage in EQ except my pet. Adds usually meant me running or dying during solo play and adds in group places usually demolished my pet in a hit, so I was useless as any form of CC. Mage was far from the worst solo class in EQ, but every class of every mmo I have played since have had massive increases in survivability compared to it. That survivability is also brought to the dungeons and group play, making it harder to design them not to be trivial the more people are in the group regardless of classes. Not that it can't be done, but it requires more time and money, which isn't usually a commodity publishers/shareholders award mmo developers.
