How is the bias current reference value determined for CMOS circuit design?
First of all, bias currents in the range of 1uA or below are used as well. It's just not that common.

The current in a circuit is determined by a number of factors, like bandwidth and noise. This results in typical currents that are in the range of a few uA up to a few mA. These currents need to be derived from a reference current and having the reference current in a similar range makes this task easier.

Deriving 100uA from 200nA would mean a scaling factor of 500. This is impractical because such a ratio introduces larger variations due to mismatch. And there is usually no point in saving 1uA when the circuit needs a few hundred uA.

Regarding mismatch in a current mirror it is also important to minimize the transconductance to current ratio of the transistor. To this end for MOS transistors a minimum current is required, to get a sufficiently high overdrive voltage.