What is the 'fuel cost' of a pneumatic braking system vs. a hydraulic one, in terms of weight?
There is an existing triple redundant hydraulic system in those planes required used for the control surfaces. Those are not something you want controlled by pneumatics.

This means the weight of the compressor, buffer tank etc. is already there and using it for brakes means you only need to pull a line from a supply/return line (which would also be near the gear to actuate it anyway) and adding a servo valve.