See, the problem is, there isn't any one way to be a best seller. Just like there isn't any one way to break into the publishing industry or hollywood (especially the latter!).
Best sellers of all kinds become that way. Some are for exceedingly great writing that have a voice that speaks to the audience. Some may not have the writing perfect but the story captures the imagination of the readers. There isn't any one subject that does it either.
If there was a formula, you can bet publishers would have it and use it regularly and predict with great acuracy which will sell big and which will flop.
Point is, they can't.
JK Rowlings advance was 2000. An advance is roughly the amount the publisher expects the book to make.
Where as some of the "big ticket" books that got million dollar advances didn't even make the advance because they flopped.
If such a formula exists publishers wouldn't be going through the guestimates and they have a heck of a lot more experience with delivering to the readers best sellers than we could ever hope to have. If they can't figure it out, no one can.
Look at M. Night Shalaman. He had a "formula" for a perfect thriller. Only two of which were block busters, The Village was just sad, and he flunked the one that deviated from it. Lady in the Water or something like that.
Sure there are some formula writers who continue to succeed book after book, but that doesn't mean their formula would apply to anyone else.
Readers are fickle and they will surprise you on what books they like and which ones they don't.
And if there was a formula, every single Dan Brown Book look alike would be a mega seller, every JK Rowling look alike would be a mega seller, but they aren't. People want fresh and new and exciting while sticking to something that they know. Some books break out of that norm, then become a new norm. What makes one book stick out more than another? The readers. It all comes down to the readers. What might be a best seller this year might have gone unnoticed in ten years. You never know.