In my school, the pay difference between universities and our college is quite vast for part-time instructors. We pay 1/3 to 1/2 what surrounding universities pay. Given that, you end up getting those who for whatever reason prefer to work for less (e.g., it's convenient to home or work) or can't get the better gigs.
That can include credentials - we can hire industry people with limited formal credentials, whereas universities prefer a PhD or at least a terminal master's degree. We can hire people who actually do things in the field. That can be a very positive thing - but it also can lower the bar even further if you'll just accept anyone.
*Most* of the part time instructors I've hired for my program are great people. There are some that are not, and you work with them to improve - and if they refuse to do that, we have not renewed their contracts.
One particularly toxic person tried to get a raise saying he had a better gig elsewhere. We told him to take it because he was a consistent pain in the a$s. We hired people to replace him mid-term because he just quit. I looked up his director at the new gig and told him what kind of person he was getting. He hasn't been employed very consistently in his life, for obvious reasons. But he has a PhD, so HR was keen on hiring him. Credentials are no replacement for being a decent human being.