Prometheus did well because some people were still expecting an alien movie, it was being directed by the original director, and there hadn't been an alien movie since 1997.
Once the newness wore off and people realized the direction Scott was going, covenant didn't have an enormous chance. That and it sucked, and word of mouth hurt it. Either way, there's not going to be enough attention and money to make a third prequel worth it to the studio. We've already discussed how Scott might have to make the third movie for even less money if he even gets to do it.
Maybe the next film only makes $130mil total. It's a lose-lose probability, and I don't see the studio taking that risk. If they do, they're crazy and I'll love them for it. But that's not what's going to happen.