I know duels went from expected/tolerated to unacceptable in the UK and US in the 19th century. However, even if duels were in a long decline before 1800, I don't understand why there was such a dramatic drop in societal acceptance that they became practically extinct by the early 1900's.
Apologies for being a bit busy today to write you an answer, but I do touch on this in some other responses I've written up, so I'd encourage you to check these out as they should be of some help at least.