I think their biggest beef is, okay you're not gonna hire me to fly Delta passenger on widget jets. So now I go back to flying Delta passengers on widget express jets. The details matter. Delta website, Delta tickets, Delta service, Delta jets. Just because they are cheap outsourced operations still doesn't change that they represent Delta's image, brand, and provide them customer service as expected per Delta standards. On one hand, they're not deemed good enough to fly Delta jets and pax. But on the cheaper outsourced side, they continue flying the same brand and pax on the (many) same routes/hubs/cities.
I'm not saying it's right. I'm just saying I can understand their beef.
I also get that regional operations are different, there tend to be more d-bags compared to mainline (partially due to younger crowd (though not all young ones of course), partially due to very minimal hiring standards, etc). And that at mainline you are flying longer flights, longer trips, and could be stuck with a for a full 14 day trip to Asia. That can't be fun. So I get why Delta (or any legacy) can turn down why their own Express/Connection carrier pilot.