Its all about intrest and schedules
Most people who get into D&D do so when they are in highschool, or University / College. This is a point in peoples lives when they are chronically broke, and have ample spare time. This translates in to many opportunities to game.
Once poeple have jobs, you see a bit of a split. There are those who will game because they find it to be an excellent use of their time. And there are those who will game when they lack the means to do something else. The people who fall into the 2nd category are still good people and good gamers, they just have other things that they can do that they enjoy more. They tend to be likely to opt out of a game for other plans. As an aside, there are aparantly alot of 'military' gaming groups. I assume that spare time comes up for these people, but they cannot otherwise go off base.
Once people have families, they end up with more obligations on their time. Adult gamers tend to have to deal with such troublesome things as thier wives and children. These people, if they enjoy gaming, are typically able to make time to play. It requires effort, but they can do it. But they cannot always guarantee that time on a regular schedule. Stuff comes up. And anyone who is married <which I am not> probably can recall instances where the spouse committed you to plans in your absence. Often, you would agree to the obligation if you had thought of it, or were asked instead of your spouse. These are the gamers who will say that they cannot free up the block of time.
But, while any individual can probably guarantee a 4 to 7 hour block of time to play D&D, it is very difficult to get to adult gamers to free up the same block of time, unless they are husband / wife.
Now, try to get a group of 5 adults with wives and / or children who can consistently free up a 4 to 7 hour block of time on even a weekly basis. No one wants to game on a night when they work the next day, unless the game is likely to stay capped at 4 hours. And weekends are often spoken for by their families.
This is why I only run my game once every 2 weeks, and on a Thursday. Its not that I think its the best gaming situation, but its one that I can guarantee with some consistency. And even that gets cancelled more often then I would like it to.
But, I am a gamer, and I game when I can. And while I cannot do D&D as often as I like, I do get to do it. And I have had success recently with getting co-workers to play Illuminatti with some frequency. The game can be run on short notice, and does not require very one intrested to be present at the same time for every game.
END COMMUNICATION