I asked essentially the same question back in June when they introduced that feature:
Swol responded with saying it was designed that way due to performance issues that would occur if they did an exhaustive ranking, since BiB needs to be run for each effect. It is definitely a reasonable concern, and for most users, they are probably only after what is the best item to put a particular effect on.
I ended up making a spreadsheet which I updated each time I got a few upgrades or upgraded my cloak, where I kept track of how much of an upgrade each effect was, and then order them by percentage to find the best one for the current set of gear. I was only doing this for one character, and ultimately, only ended up doing it for my main spec as BM, so the choices were actually fairly simple and straight-forward. It would have been more of a pain if I was more active and needed to do it for my alts as well.
Considering corruption effects are going away in the pre-patch (as far as I know), which is probably only 2 - 4 weeks away, I don’t see them making any changes to this feature, since it works fine for the way most people use it. Their primary focus at the moment is preparing AMR for Shadowlands, which I think is much more important.