The reason why we don't have a way to say "Only show me books from my To-Read Pile" in the recommendations filter menu is because there's no way we can guarantee high quality recommendations for everybody given the matching up between reading preferences (which could be anything) and the books on someone's TBR (which, again, could be absolutely anything).

This is why we developed the Up Next Suggestions feature — it's a way for us to make high-quality recommendations from your TBR (we still combine machine learning models with most of the categories to choose the best pick for you in a range of the categories) irrespective of your reading preferences.

This is not to say we won't eventually have this option regardless (and just a "there are not good matches fitting your criteria right now..." notice), but I wanted to explain the reasoning. :-D