It's because they're the worst pilots and they have the highest failure rates of any demographic of pilot hired at an airline. Period.

The 91/135 world is a wildcat arena that lacks standardization, SOP's, or any real training. The biggest challenge at FlightSafety, SIMCOM, or CAE is getting your check to clear.

Take it from a former 91, 135, 91K, and sim instructor who did manage to get to an airline when I tell you that that hurdle is higher for non-121 applicants and you're far behind your 121 regional peers. It will be harder for you to get hired and you'll struggle in training. It isn't impossible but it is far more difficult.

If you want to work for an airline then go to work for an airline.