Since much of the airline pilot profession, especially Legacies, is dominated by ex-military guys, some would consider it sacrilege to force a new-hire to go from F-15 to a Beech 1900.

As far as RJ's...maybe they're afraid management, in an effort to maintain CASM would make mainline guys take a payscale to fly RJ's that resembles the "industry leading" and precedent-setting 70/90-seat scale negotiated by a certain airline.