Unlike the 170/175, the CRJ doesn't switch between HP and LP bleed according to thrust setting. For ECS, all you have is 10th stage bleed air, or the APU. Idle 10th stage bleed air does nothing on the CRJ200, unfortunately. The only way to cool the cabin on the ground is with the APU or increasing the thrust significantly to supply pressure to the packs. For obvious reasons, it is not practical to run the thrust up on a taxi-out to get the necessary air flow, so many just opt to run the APU until just before takeoff.

In the case of an APU deferral, the cabin temps can soar to 35°C+ very quickly with 50 pax onboard and no airflow with an OAT as low as 80°F.