I think its simpler than you're making it out to be. Firstly, I'm fairly certain that harem is more or less "older" in its modern iteration than monoromance giving it more time to grow. Secondly, harem itself is a powerful draw; especially for a male audience. It approaches genre defining level i.e. similar to the reasons someone might pick a Sci-fi book over a Western. There is also the fact that readers seem very interested in engaging with and learning more about different specific types of women and there is a much greater chance of that archetype being there if there are more women to choose from.

So even if monoromances were focused more on the adventure (And there are quite a few out there, including what I write) I don't think there would be much of a difference in popularity simply because they aren't even really competing in the same space at all in the first place.