I'm not familiar with this category of motorsport, but at least in F1, all safety systems need to allow the driver to leave the car in less than X seconds in case of accident (for example, if the driver needs to get out of the car in the case of a fire).
So maybe, and this is a guess, airbags are not activated in lateral impacts like this one as they could make this escape more difficult than the damage they are preventing.
So maybe, and this is a guess, airbags are not activated in lateral impacts like this one as they could make this escape more difficult than the damage they are preventing.