With the COVID vaccines, the locations of the clinical trials were chosen for how rapidly the virus was spreading. Some trials even added new geographic areas with high transmission rates while they were still ongoing. Why wouldn't any large scale HIV trial not choose one of the many countries with a high transmission rate?
That's for phase III trials, which I am sure will be conducted in countries like South Africa. This is not there yet, and phase I safety trials are easiest to conduct in the vicinity of the researchers and good hospital infrastructure.
It is maybe that Moderna makes a partnership with larger drug companies for phase ii iii trials. Also maybe they try to conduct in subsaharan aferica where there are large spread rates among straights and disease is more endemic in populations.
Phase 1 is just a safety trial. Imagine how many years it will take to make a statistically sound conclusion of the efficacy rate?