Here's an AP story discussing emotional attachment and games work together (Thanks VE) and of course how this translates to the bottom line. There is also a bit in there about how the face of gaming is evolving and some speculation on what the catalyst for it all is.

Some software designers attribute the changes to an aging market that's careering toward a third decade of gaming. Others say this simply the technology evolving naturally, that more computing power means pixilated heroes and villains can be smarter, faster and vulnerable. In short, more lifelike than ever before. [...] ``Players have experienced in-your-face shooting in the past, and now they want more,'' said Todd Hollenshead, chief executive of Id Software, a developer in Mesquite, Tex. ``Call it 'been there, done that.' Players are looking for the next thing that is going to wow them.''

I'd say it has a lot to do with the original computer/arcade gamers getting older and more mature. Kids naturally gravitate towards new things so when arcade gaming started out it was easy to target that age group. Now 20 years later those kids have grown up and still play games. You've got a much wider demographic now. Seems simple enough.