It's a performance issue.
I'm using a machine with medium-range spec, which has some trouble running IL2/FB at full settings. When I turn the mirrors on, you can immediately notice the drop in framerates.
Basically, a mirror doesn't work in the way we think it would, in a virtual world. It means by implementing a mirror you actually create another 3D model at the mirror region, and make them work in relation with the movements of the plane behind you. So it's double the workload.
If, there is more than one plane to depict in the mirror, it could easily become triple, quadriple workload.