The answer, in fact, is simple: there is no radio communication with Earth on the other side.
This problem can be solved by grouping satellites that will forward the signal, but it is expensive and difficult. And without constant communication, putting people on the moon is an excessive risk.
Of course, now I’m talking about manned missions. Automatic stations, of course, have been working from the «back» side of the moon for a long time. For example, the Soviet automatic station Luna-3 was the first to receive her pictures.
