To add another data point to this thread: I just got accepted for an all-areas free Bookbub for a book which is in KU. So it can be done, although I think a lot of it is about genre. In Regency romance, there are typically two offerings a day, one free and one 99c or more. That means twice as many slots as in some genres, and they're more likely to have a lull where they're happy to take a KU book to fill in a gap.
Interestingly, although it took me 60 submissions to get my first Bookbub, it got easier after that. At one point I had five straight submissions accepted without a rejection. It's got harder the last year or two, though. This time I had two rejections before they accepted one. They turned down my two never-before-discounted offers, one of them a good value box set, in favour of a book they ran just over a year ago. So I have no more idea than anyone else why they accept one book over another. I think a lot of it is just happenstance - catching them on a day when yours is the most appealing option. It has *nothing* to do with the quality of the book. Good books get rejected multiple times, less good get accepted, all without rhyme or reason.