Why is it desired to divert the surplus PV power into a resistive load?

Spacecraft aren't air-cooled. Simple as that.

Solar cells operate in full sunlight, and get hot. Dissipating 30% of the incident power somewhere else helps keep the temperature manageable.

The unlit side tends to get very cold, for much the same reason - so there may be benefit to using the heat in another part of the satellite.

Solar cells in open circuit while in sunlight as done in a typical MPPT charge controller will heat up faster, and their efficiency will drop compared to a typical shunt charge controller setup. Shunting the excess energy at the ideal power point will maximize the power usage while minimizing the heat they need to manage. As cooling is more difficult up there, this is the preferred setup. Their (ISS) on/off cycles and load needs are also different than typical earth systems so every bit helps.