I think part of it is that at CC's, teaching is literally the most important part of the job, as opposed to at 4-years, where professors are supposed to be spending the bulk of their time research and teaching a class or two on the side. As a result, at a CC, teaching is the number one qualification hiring committees look for, but universities are usually looking for who has the most potential/publications/funding as a researcher with little to no regard for their ability to teach. As a result, unis get people who care more about research and regard teaching as a distraction (IME).

And I really appreciate that you have an understanding of the plight of adjuncts and other non-tenure-track instructors. There is some movement at the CC level with several big unions actively working to advocate for adjuncts despite the fact that the adjuncts are not explicitly part of the union (generally due to explicit exclusion or an inability to afford dues), so hopefully that gets better soon!