Dear Miss Snark,
I've searched and read your blog for references on backstory, and it's evident that you much prefer action in the first few pages of fiction.
Surely, all agents (and publishers) can't share your aversion to a novel that begins with backstory, or writers like Anne Tyler, William Trevor, Anne Lamott, or Anita Shreve wouldn't succeed.
Appealing to your sense of reason, can you explain why backstory clearly has worked in so many cases? Although it may have been addressed somewhere in your blog, I haven't found you to point out an example of backstory that works in an opening. Is it simply not your taste in opening a novel?
The writers you cite are wonderful.
A lot of the writers in my slush pile and Crapometer aren't.
A lot of them are unpublished, and seeking representation for their first novels.
I don't think it's bad to open with backstory; I just find most people can't do it very well.
Think of writing a novel as a gymnastics or diving competition. You can attempt a triple back flipschitz with a 3.0 degree of difficulty but if you go splat, you get a major deduction or disqualification. OR you can try a double back flipschitz with a lower degree of difficult, execute it perfectly and win. I'd like you to win even if you aren't a triple flipschitz quite yet.