NCD,
I think at the time the Dash8-400 came under Dickie Smith affordable safety/statistics ramblings
A concession was granted ?? on the basis that the Dash was capable of flying BNE to Norfolk, missed approach followed by engine failure and depress and then carry on to the approved alternate aerodrome
Airframe was a proven quantity, engine ticked all the boxes so to speak
Regardless of the comfort factor of flight crew, the bean counters love it when more paying pax sit on bums!