Porsche built an airplane engine in the 1990s that used a single throttle which was briefly available in a Mooney and as a retrofit.
Some modern piston aircraft engines use single lever control as well.
While a single lever control is feasible there are a lot of reasons why pilots might want to be able to control propeller RPM independent of actual power demand.
IMHO the Dash 8-400 setup was chosen to allow for maximum similarity with the Dash 8-100/200/300 arrangement. The condition levers are not really necessary as demonstrated by the Fokker 50 which had power levers and small ON-FEATHER-OFF selectors resembling a jet layout. Power Rating and Propeller RPM setting were controlled by pushbuttons (TO, CLB, CRZ, MCP, GA).