Seems like the FO's that I've flown with that are leaving or trying to leave are going to Jetblue. I'd say that's for lots of reasons, several of them live in Florida and want a major with a base in Florida, but also, as mentioned above, for an FO, even first year at Jetblue is a pay raise. Every year after is a pay raise above what they'd make as a Captain here. If they're looking at another year to Captain here, why not go there for a raise now and at least lateral (and likely above EDV rates) every year after?
As others have mentioned there's a certain amount of stability and additional quality of life to be gained at ANY major (even those that aren't legacies) vs a regional where we could lose our flying at any point.
Also, with the DGI running about 50% and Delta being more than willing to hire Jetblue FO's, there's really no downside to them. They give up their 50% chance 18+ months from now, for a guaranteed raise now, potential to move up at a career airline and they still have the ability to put an app in to Delta in 18 months and they might still get called.