Numerous FO's that I have talked to say that quality of life issues are the basis. A large percentage of the senior FO's in the company that have all the requirements are in the jet. The RJ is easier to fly, more time off as a mid range to senior FO than as a junior Captain on the 120. Many recognize that they dont need the PIC in a hurry to move on, as there is little movement. A jr captain on reserve doesn't make that much more than a senior RJ FO flying a productive line. I know, I was there a few years ago. Extra couple of bones a payday wasn't worth commuting, etc.
I believe DEN is the jr domocile for the Brasilia, but thats a guess.