It's easier and cheaper to test new drugs outside the us/eu because laxer regulations.
Given your comments elsewhere I suspect you're claiming this with no evidence.
For other readers, this isn't true / it's drug dependent. Amongst other things: difficulty sourcing the correct patients; difficulty importing drugs, particularly because they're not yet approved; difficulty getting high quality or reliable labs; etc. As any ctm could have told you.
As for this drug: the US/EU don't have rampant hiv, esp with the use of PrEP.
The reality is that most clinical trials aren't successes. If you can get a huge cohort of people for relatively cheap elsewhere, you can screen a lot of promising but doomed tests at a cheaper price point, then only re-create similar testing on the most promising candidates in your lucrative markets.
There may be common reasons to trial there like it being cheaper or less regulated. But there is a good reason for this specific medication to be tested in those specific countries. Criticizing the study authors for being "cheap" is uncalled for in this case.
[0] https://ourworldindata.org/grapher/share-of-population-infec...
Hard to say, maybe it's not inconceivable that ~1% of potential patients in the US/EU/etc. might end up paying more than > 50%-90% of the people living Sub-Saharan Africa for whom getting the vaccine would make a lot of sense.