What differences in traffic patterns account for the fact that STDM is a cost-effective form of multiplexing for a voice telephone network and FDM is a cost-effective form of multiplexing for television and radio networks, yet we reject both as not being cost effective for a general-purpose computer network?

Answer explained below

Explanation:

statistical time division multiplexing (STDM) and frequency-division multiplexing (FDM)

They both STDM and FDM works well for the channels which transmit continuously like telephone, FM radio and TV which continuously transmit signal. Since inactive lines are wasted this makes these venues a good fit for STDM and FDM. which means if the signals are dropped or not received to the receiver than it won't have to send the signals again which is why STDM and FDM are useful in these cases.

In Traditional Computer Communications system, the system have tended to be a lot more 'bursty' with idle periods. Hence if we use the FDM and STDM than the usage patterns of STDM and FDM would waste this idle time. so that is why we tend not to use the FDM and STDM in the general purpose computer network.