Depends on the regional I suppose. Envoy, well this could be a long list but I will just start with a few.

1. Significantly better pay.
2. Significantly better qol
3. Significantly better schedules
4. Significantly less hostility from management
5. Significantly better union in terms of being able to actually defend their pilots. Our current MEC is doing a fine job, but we only have so much power at the end of the day.
6. Better jump seat privileges on the routes that actually matter.
7. Duty Rigs
8. Min Day Guarantee
9. Better per diem
10. Meals

I could go on. Its not worth it though. Mainline is far and away the better option.