Why aren't magnetic substrates so used for RF printed circuits?
But why magnetic materials are rarely used?

A few thoughts: -

Extra cost
Extra losses (eddy current losses especially as frequency rises)
An extra process
Another variable
Changes in permeability with temperature
Changes in permeability with frequency

Another thought

Having a magnetic substrate across the whole PCB will mean extra undesirable properties on PCB tracks that should not have inductance increases. To avoid that would mean that the magnetic substrate will need to be implemented in some areas and not others.


The only benefit I can envisage is that you can thicken up tracks for 50 Ω impedance and get more current throughput. Or, you can make boards thinner whilst still maintaining the same characteristic impedance.