I've been thinking some more about this. There weren't that many Neanderthals. The population size varied around 10.000, 70.000 tops.

For sapiens, in those times, it was likely similar.

So that makes interbreeding even more rare. It wasn't like they met each other a lot. So this means that it happened only one time that interbreeding happened and was successful in the sense that it produced offspring that reproduced for millennia.

So regardless of what possible biological outcomes are, rare events can produce odd results even if there were more possibilities that could have happened too.

I also read about evidence that this interbreeding could have happend way earlier than previously thought, and that the neanderthals that spread through Europe were already a human neanderthal hybrid. This could have been a one time event, rather than a series of events.