AAV based therapies may have no permanent effect when the cells in question are actively proliferating (and the payload dilutes with each division) but muscle tissue is largely post-mitotic.

mRNA is in comparison very transient (in the range of days, and that's being charitable), even when modified (5' cap, uridine analogs, poly(A) tail) as it was in COVID vaccines. This is fine for vaccines, as you essentially want just a single exposure to the protein with each vaccine dose. You do need dystrophin continuously though (even though the cells are not dividing much, they are still recycling it).

You could argue for delivering gene therapy with mRNA/NLPs in multiple doses over the course of patient's life but that would likely 1) exacerbate toxicity and 2) be super-expensive