The simple reason is he doesn't want to be executed . mOO
He would have easily been able to work out a plea agreement that prevented him from being executed if he had confessed to these additional victims when he confessed to the others. I get that these are different jurisdictions, but there's plenty of examples where murderers in the same situation have been able to avoid the death penalty through confessions, especially when there is still an unrecovered body involved and the murderer agrees to reveal the location where it can be found. That would probably hold true even today for these crimes, especially considering his rapidly declining health (apparently), age and current prison sentence.
As someone who started following this case so closely back when he resurfaced that it bordered on obsession, my initial reaction to all of this is that there are no additional victims and he's just enjoying all of this unexpected attention. We've known all along he did drawings of women tied up in various positions and was obsessed with the weird detective magazines back in the day that included similar sketches. I've never heard of there being drawings located that depicted any of his known victims, rather the only ones I'm aware of were basically the kind of thing you would have seen in those magazines with "generic" women. I have a feeling that these newly discovered sketches are the same type of thing.
I wasn't even aware of the fact that these possible new victims were sexually assaulted through actual penetration/forced touching, whereas none of his known victims were (as far as we know), even though there was obviously a huge sexual component to his crimes. This makes me doubt the new victims even more. I know killers can change over time, but this is a pretty integral part of any serial killer's operation and doesn't seem like something that would be altered in this direction after happening during multiple murders. I could believe it changing in the other direction since it's not uncommon for offenders to work up to certain acts (like rape), but it seems much less likely that he would go in the other direction, especially since he was still a relatively young man when he killed the Oteros. I could maybe even buy that he didn't assault Josephine because of her age, but that doesn't explain the later victims.
If he is responsible for these other murders then I hope they find the evidence they need to prove it, but until I see such additional evidence then I'm going to keep leaning toward them not being his victims.