Because HIV is a retrovirus, that copies code into your DNA. But it is a single-stranded, positive-sense RNA virus, so maybe if Molnupiravir does work against the other positive-sense RNA viruses, it will also work for HIV before it can get into the DNA. Or maybe HIV victims will have to keep on taking it for life, like the current HIV drugs. Nobody knows yet.
Working for the positive sense viruses may not be possible, if the direction of replication is a big part of the drug's action. The viruses supply their own enzymes that construct the replicant strand of RNA either from the 3' end or the 5' end of the original strand, but not both. At that level, a lot of stuff happens or doesn't happen because of mechanical constraints. Something as small as a left-handed or right-handed amino acid enantiomer can screw things up.