The reason is the plane changes altitude very frequently and very fast, with its paraboles to mimic 0G. Because of this, it is necessary to find out if the body can withstand fast depressurization.
When you look at the people experiencing the paraboles, they often chew a gum. That's because they need to pop their ears up when they go through the altitude changes. Of course, planes are pressurized but with altitude changes, there is still a change in the pressure (I've noticed that in normal commercial flights). If there's a stronger loss of pressure, then there's a problem but it can be overcome, provided the medical tests certify it.

Raoul