[kozinc]

Being a layman too, as I understand it, it's because the deafness is caused by a single (mutated) gene which makes it (comparatively) easy to change. In this case, because the altered gene causes a necessary protein to be faulty or missing, gene transfer therapy can introduce a normal copy of the gene to recover the function of the protein.

For other diseases caused like this you're mostly right on the money - sometimes because while a single gene might be the culprit, the mechanism is unknown, sometimes mutations in some ‘single-gene disorders’ may not be in a single-gene at all, not to even mention other possible interactions or the risks inherent to gene therapy.