Pneumatic have big advantages compared to hydraulic in this specific case in that the air pressure represents a spring and it is easy to measure the air pressure to protect from hurting. And the same air pressure can then drive a number of rods at the same time - all with same pressure.

Since hydraulics don't have any springiness, you have a harder time to make sure that you don't push too hard. Every ml of extra fluid just have to push deeper/harder. The pump must be able to supply enough oil to handle multiple rods going deep but without hurting. And if the patient for some reason tries to move the leg, there is no spring effect to protect.

It isn't so expensive to build a pneumatic system since it doesn't matter if there are a little bit of leak. And the forces are low, so no special tubes are needed either. Even hobbyists can make cylinder/rod designs quite easy.