One of the things with fitting hearing aids nowadays is that they are supplemental to your remaining natural hearing. For “traditional” sky-slop losses this is relatively easy: make a vent in the ear mold to pass through the natural sound and add the high frequencies that are needed to compensate for the high frequency loss.
With a reverse slope this doesn’t work, because to boost the lower frequencies a larger contained air mass is needed in your ear canal.This prevents the use of a large (or any) vent. Otherwise all the amplified low frequencies would just leak through the vent.
Unfortunately this leaves no room for keeping the natural high frequencies passing naturally and with that your entire sound perception will be through the hearing aid. This means that you may loose all hight frequencies that a hearing aid simply doesn’t pass through (roughly above 8-10 kHz) and also that any directional information is lost. Therefore this is much harder to fit than the sky-slope.
I’m not an audiologist, so correct me if I’m wrong please, but this is the easy explanation as I have understood this matter.