I think there are four main reasons why the genre has gone from a focus on social and team-based gaming to one that caters more to the solo gamer: Hero complex, casual audience, developer greed, and social behavior.
It is human nature to want to be the center of attention or at least feel like the hero on some level. It's also not too far of a stretch to call members of our species generally selfish. How can you really deliver this experience if you force your players to ask for help all the time? I think this was simply a natural progression of the genre in trying to appeal to our natural traits.
This, however, leads to the bigger picture of trying to expand audiences for the genre. Early MUD and MMO players could probably be considered the most hardcore of the hardcore gamers back in the day. Over time, those gamers grew up and got responsibilities.
Many hardcore gamers became casual gamers. Add to that the millions of potential casual gamers who have never played an MMO in their life and all of a sudden your potential audience is huge... as long as you make a design shift toward more casual- and solo-friendly play.
Finally, I believe it all comes down to the mighty dollar. Audiences grew and so followed the market and competition. Suddenly, you couldn't make MMOs on the cheap anymore (though a stalwart few still try).
Not only are game studios focused on appealing to the solo casual gamer to maximize earnings, they also want to build in artificial time sinks to make players stick around. Bind-on-Everything gear/tokens. Vertical expansion. Daily quests. Random loot. Reputation grinds. Dungeon keys. Etc. etc. etc.
Simply put, highly social gaming is being killed by a desire for mass market appeal and the mass market has an insatiable appetite. It's no secret that we live in an instant gratification society focused entirely on ourselves and our very small groups of friends and family. As the sphere of the MMO audience expands, is it really a surprise that larger societal issues influence the genre?
Just like society, MMOs are evolving (or devolving, depending on how you look at it).
How often do we band together on a large community scale to help out others in need anymore? How many barn raisings -- so to speak -- do you see these days that weren't inspired by some tragic catalyst? How many people sit out in their front yards talking with random passers-by just for the heck of it? Do you even notice the people around you on the subway or do you bury your head deep in a newspaper or magazine with ear buds equipped to ward off anyone silly enough to even think about distracting you?