A disease is a good fit if you can just inject the therapy into a localized region (say, the eye, or an organ) and the treatment works for a reasonably long period of time (months+). Typically another requirement is that the target is a defective gene where the phenotype can be repaired through addition of a "corrected" form of the gene, without the defective gene needing to be removed. This is the case in X-linked retinitis pigmentosa, where the genetic cause of the disease has been understood for some time, it's relatively simple underlying mechanism (so we think), and you can deliver the medicine to the retina with periodic injections.

Treating a disease where you have to remove an inserted retrovirus from a large number of freely circulating or "hidden" cells (which is the case in HIV) is far more challenging- you need a way to recognize the cells of interest, access all of them, and get 100% transversion. All without causing negative side effects.