With an antenna of much simpler construction, cheaper to install and maintain, diversity reception using two rhombic antennas could be introduced, improving the reliability of circuits. (Say a bit more about diversity reception)
Although perhaps not possessing the gain of a single frequency Beam antenna, it was a good compromise between cost and complexity and with higher power output from valves being possible by then, the ERP lost in the antenna could be added in the transmitter.
This picture shows nested multi-wire Rhombics, a fairly typical implementation in later years.
Multi-wire helps to stabilise the impedance and the nested pair of antennas enables optimum take-off angle for the lower frequencies (bigger and higher).