Why a PMOS active load is used for a NMOS common source amplifier?
diode-connected FETS unless very long channel will be high-GM, low-incremental resistance, causing low gain and high distortion.

Using the Pchannel as  active-load may result in very high gain (depends on the channel length, so lambda-coefficient is not a bother) and low distortion.

In any case, long-channel FETs have higher channel-bulk capacity that has to be charged and discharged.