I don't know all the reasons, because I don't know all the adjuncts. But I know some.
Let's be clear about whom we are speaking. We are not talking about practicing professionals: doctors, dentists, lawyers, businessmen who come back and do some teaching, of various time commitments. Such folk have a variety of reasons for doing this, some reasons are even altruistic. But none of the reasons is money. These are the folks who make real money at their day jobs.
What we're talking about the ABD's, the recent grads, the young people who work for something like $3-4K per class, and given their hours, they make less than minimum wage.Indeed, most of the these folks would jump at a TT job. Most of these folks have been trying to get a TT job, and send out reams of applications, while trying to publish just one or two more papers. And, yes, an adjunct position is definitely a second-best option for the people I know.
So. A comment said that adjuncts are paid in a false coin: the promise of it being a stepping stone to a "real" job.
I disagree. There may be hand-waving and vague comments in that direction, but nothing substantial. Nothing that smacks of "promise".
Many of the people in adjunct positions that I knew/know, both IRL and in the blogosphere, have other considerations that prompted them to take an adjunct position: family issues (spouse, children, parents) that keep them from being able to take a job in the hinterland, a commitment to living in a Certain Place. Some I've known are married, with kids, and struggling to finish a PhD with no support, and need/think they need a job. Of course, people who can afford to stay in a postdoc position, often do. It certainly pays better than adjuncting. And I have seem a few, by and large single, white, male, footloose and fancy-free, who can't find anything else, and are willing to try to stick it out in the system for a bit longer to see if they can get a job.
I know I sound like a broken record, but I think there are two things operating here, the first of which is choice. No one is holding a gun to anyone's head and saying "I will blow your brains out if you do not take this adjunct job". There is lots of information around about alternatives, columns in SCIENCE, and internet resources that did not exist 20 or 30 years ago. There is more than one choice being made here. Choices that say: I don't want to move, I've commitments to this geographic area. Choices that say: I do want a SLAC, I don't want a SLAC, I want Ivy League, MRU, or I want to be in A Big City.
Some of these considerations are not frivolous, and they are valid life choices to make. We each decide what is important to us, and frequently our decisions look irrational or stupid to someone else who has made different decisions. I'm not saying that its right, let alone a good thing, to make people balance two careers, to make people choose to be near family or take a job somewhere else or to ask a person of color, a LBGT person to move to place that is blatantly hostile to who they are, just because that is the only job there is for them. I also know that a choice to stay where one's spouse has a good job is a very different thing than the dilemma of a POC or LBGT have to make. My point is only that there is some choice operating here, and no one takes an adjunct position without being aware of those choices.
The other operational consideration here, one more time, is too many mouths at the trough. If you are a faculty member, and over your lifetime, let alone right now in your lab, you've trained more than 1-2 PhD's who go into research, you are not part of the solution. This is true even if you are the Most Important BSD doing research to cure cancer, make disable children walk, or solve Global Climate Change. The reason there are not enough jobs, not enough grants, is because more people want these things than are available. Even if grant money flowed more easily, more frequently, and in higher aliquots, the positions available would be soft-money ones, that depended on continual funding, and not tenure track. It would be a marginal improvement, in terms of salary, but not in terms of security and future. Universities are not about the expand the number of TT positions.
So we've got adjuncts. Make sure your trainees know the score. Make sure the trainees in your department know the score.