Because there are presumably many well-known moments, and this one seems to be the one most people seem to remember. A best-known moment sounds odd, IMO, and probably would imply that among all the moments studied by people who study this sort of thing, this one they know the most about. Personally, I probably would have written "the most famous incident/moment..." or entirely rewritten the sentence saying something about "he most famously did this or that ..."

Interesting question.

The most likely answer is probably that 'well-known' has just become a fixed, invariable adjective, so no one stops to think about its constituent parts, about deconstructing it and reconstructing it in the superlative.

I'm inclined to agree with dude that it reads a little awkwardly in writing, but on the other hand, I don't think it would catch my attention at all in conversation. I might say it, certainly.

There may even be some logic to it. To me 'best-known' could sound more like it was judging the quality of the knowledge or learning. For example, if you knew Russian somewhat, Latin fairly well, and English excellently, then English might be your best-known language -- the one that you know best.

In contrast, English is probably the most well-known language in the sense of a lingua franca -- most popular, known by most people.

I actually have trouble translating 'popular' (bzw. 'well-known') E>D, because 'gut bekannt' doesn't always seem to fit into the syntax I want to use. Maybe this kind of thing is partly why.

I think I understand it a little better now. Thank you both!