Why do localizer/glideslopes use 90/150Hz specifically?
The frequencies are chosen to allow the system to produce a localizer and glideslope 'beam' that provides proportional guidance that can adequately guide an aircraft on the approach.  

Read through this Question and Answer.  

As you can see in the description of the beam-forming, the width of the antenna array and the choice of the modulation signals (90 and 150 Hz) drive the ddm proportionally to the angle of approach to the antenna array.  

There are also considerations tied to the broadcast signal. Higher frequencies would require more bandwidth which would impact channel spacing.

Ultimately, the choice of the two frequencies is an optimization of a design for the approach guidance. For any given runway the current system can be set to provide consistent guidance (+/- 350 feet == full scale deflection) at the runway threshold.  This is done by adjusting the distance between elements of the antenna array.