I got to my thirties before I realised that didn't used to was grammatically incorrect. Typically, it was in an English class that I was teaching at the time! Didn't use to was in the textbook, and it looked utterly, utterly wrong without the d, and it wasn't until talking about it with a fellow teacher (in her 60s) afterwards that we realised that grammatically it had to be didn't use to. It was one of those 'no way!' moments of revelation about the language you've been speaking and teaching for decades.
I reckon - Bion an obvious exception here! - that the vast majority of people simply don't know that the 'correct' form is didn't use to. I can well believe this is true of Rankin and his editors. And I don't mean this critically. I think it's just so widespread now that unless you have reason to be confronted with the 'correct' form (in an English textbook, someone correcting you, or in amw's case in #3, by reading a Leo thread), that you're not going to think twice about it.
That's an interesting statistic from Garner quoted in #10: that didn't used to is 'twice as common in journalistic writings than didn't use to'. That means two things:
(1) that there's a large number of journalists - better wordsmiths than the average Joe (but not necessarily the average Leo contributor!) - that have also never thought about the 'correct' negative form (or they have, and have chosen to stick with the more 'idiomatic' written form),
(2) the 'incorrect' form is now more prevalent in writing than the 'correct' form, and that's only going to reinforce its dominance.
I know the 'correct' form, and probably very rarely write it, but I still picture it in my head with the final d.