The CRJ and F70/100 were developements of existing rear engined jets…Challenger/Fokker F-28 where rear engined was the way it was done during their development and in the case of the Challenger, the style for BizJets, which is still the case.

Newer Regional Jets seem to favour the underwing mounted.

It is easier to offer different engines on under wing pod mounted as rear fuse mounted does need more engineering to change.