Their multiengine minimum is too high. A new pilot who can't afford to buy 300 hours of multi time is going to have to either get it flying Part 135, or by teaching in a twin. The Part 135 job is going to pay as well as the airline job so that pilot does not feel a great need to quit for the airlines. The instructing job would take 2-3 years to get to 300 hours of dual-given so that pilot is either busy building multiengine time or more than likely can't find a job teaching in a twin in the first place.