I think it's a combination of things.

First, as much as people complain about a bad service experience, when the next purchasing decision comes along, $$$ seems to be the deciding factor. Some people are at places in their lives there this doesn't matter as much (and fortunately, I'm gradually getting there), but the reality is that most aren't. Thus, they look at it in terms of: "If I spend the $100 extra to go on a different carrier, I won't be able to go out to as nice a restaurant." I call this the Wal Mart phenomenon: Nobody really likes to shop there (I mean, really!), but they go because they feel it'll maximize what they can do with what they have.

The other part might be availability of options. Unless you live near a place like New York, Chicago, LA or the like, the range of options that get you where you want to go when you want to go there can be limited. Or even if you technically have options, they may involve more stops, bad connections, etc. If they flew airline X before, chances are that part of the equation was that they were available and convenient.