The term "Massively Multiplayer Online Role-Playing Game" is quite a mouthful, and it's becoming increasingly erroneous. The "Online" part still holds true, but the amount of instance-based gameplay is making them more intimate and less "Massive". The "Role-Playing" part has always been a hard sell, since in most cases the role-playing ends when you exit the character creation screen. And even the "Multiplayer" part of MMORPG is a bit misleading, since such a huge portion of the player base ends up playing alone.
The early games were specifically built around working in groups. (And by "early games" I'm talking about Everquest and the like. I'm not even going to get into the MUDs that came before.) You hooked up with strangers, you went into a dungeon, and you farmed monsters until your mouse button wore out and your spouse left you.
But some people wanted to play alone. They wanted to play alone so much that they would forego joining a group and chip away at low-level mobs all by their lonesome, even though they could make far better progress with the help of others.
Developers and group-minded players were confused by this. Here was a portion of the player base that ignored the central conceit of the game (multiplayer) and at the same time accepted less reward for their efforts. Why are you playing an MMOG if you want to be alone? Why don't you just play a single player game? But whatever the reason, the demand was clearly there for games where grouping was optional and games have been gradually becoming more solo-friendly since then. This raises two questions:
1) But who are these people who play alone?
Well, the condescending answer to the first question is that these people are the "casuals". (It's common for old timers to say the word "casual" with a twist of the lip, the way you might say "sewage" or "plague rat".) They're supposedly bored lonely housewives who want something to do while watching their soap operas or reality-TV catfight shows. I don't know how much truth there is in this, and I think most people are overlooking a huge demographic: People who are playing from the office. If the traffic at my website is any indication, people spend a lot of business hours doing stuff they have no business doing.
But whoever they are, they clearly want a low-key game that requires only moderate levels of attention. (The fact that hunters in World of Warcraft - a class that is the definition of "casual gameplay" - was the most popular class before the Death Knight came along, is proof of this.) It's nice to have a little something running in the background while you watch TV, work on spreadsheets, or write term papers. The gentle solo gameplay is perfect for people who want something more satisfying and engaging than Minesweeper but don't have the time or attention to play a full-on single player game.