I haven't submitted often--and BookBub helpfully keeps a list of my submissions, so now I know it was a total of eleven--and they've passed on this next book twice before. I think Simon has the right of it and their people pick and choose based on what other books they're got lined up.
My assumption is that the trad pub women's fiction, with its typical $1.99 or $2.99 as the discount price, is the preferred deal for BookBub. Since there is plenty of trad pub backlist to pump through the system, I doubt I could ever get a U.S. deal. I'd do a 99-cent deal and accepting my book in place of a trad pub book means $751 or $1522 in lost revenue for BB. I can't see why BB would go for that.
On the other hand, if BB wants to keep giving me international deals, and the ads each make a small profit, then I can use that profit to fund other ads that might be even more profitable. At the moment I'm soured on Amazon ads and not ready to try yet again with Facebook.