In addition to other useful comments and answers: in my context, of mathematics... : yes, only a very small fraction of "adjunct" teaching is done because of lack of expertise of "regular faculty". Examples would often be "financial math" or "actuarial math". Far more typically, adjuncts teach very low level math. Now, on one hand, while the mathematics itself is very easy, reaching the audience is non-trivial. Full of pre-existing neuroses, etc. Although the typical adjunct teaching such things has very modest mathematical ability, that is more than sufficient, and, typically, such a person's ability to "connect" to "normal" kids who're "having trouble with math" is greater than that of talented mathematicians. (Tho' not always.)
True, "The Market" observes that there are many more people able to do this than the number of jobs, so the pay is depressed. It doesn't help that there is a mythology in (academic?) mathematics that teaching itself is something anyone can do, perhaps after one has lost the "zip" to "do research". All the more ridiculous that this mythology exists among people who's teaching is awful, at every level, their whole life. Luckily, their job description emphasizes "research".
But the mythology, seemingly confirmed by The Market, marginalizes (non-specialty) adjuncts. At my current institution, none of the (non-specialty) adjuncts has a Ph.D., which further reduces their status.
And then there is the current budget squeeze on universities... Everything has to be done more efficiently, etc. Departments' supply budgets are cannibalized to pay for office staff, etc. It is crazy. Night-school classes, once paid for through separate budget lines, have been "in-loaded", so have to be covered by departments often with the same budget as before (!) So, hardly the time to think about equity for people who're willing to "work cheap".
The AAUP has long argued for better treatment of "adjunct faculty", but harsher economic times are not fertile grounds...
For that matter, often the real competition for adjuncts is grad students as Teaching Assistants, who are "more expensive" if their tuition is included in the package. Thus, at best, adjuncts have some incentive to keep their pay below that of grad students + tuition. A crazy dynamic.
It is true that the volatility of enrollments gives management incentive to find a way to avoid liability... but in the dim past there was simply consistent excess capacity, not so much a population willing to absorb that volatility!
Nowadays, upper echelons of the university almost make it against-the-rules to cushion people (other than tenured faculty) against volatility...
Not a happy situation.