Not much insight offered, but the Dash APU is simply not certified for inflight use. I don't know what extra hoops are required for that but DHC probably felt there was no need to certify it as it's not really required for any redundancy.
On the Fokker 70/100 and CRJ the APU generator is used to provide redundancy in case of failure of an engine driven generator. The Dash 8 has four engine-driven generators anyway, two of which are sufficient to power all electrical systems (sans Galley on the -400 if I recall correctly).
Bleed Air (which is
AFAIK provided by the APU on the CRJ for anti-ice on approaches) is not an issue on the props with their deicing boots. So why should DHC bother to do extra certification work on a system that is considered a subsitute for GPU and ground Airco and that is often U/S even when used on the ground only (on the -400 the APU generator seems to be too weak for even normal ground use).
The APU design is BTW completely different on the -100/300 and -400. The -400 has the APU in the very end of the tailcone while the older Dashes have it in the rear fuselage.