IMO it's not much of a mystery. For ~15 years "object oriented" was the biggest buzz word in computer science and OCaml was object oriented, while SML was not. At least in the objects, inheritance, and polymorphic way people wanted.

I learned ML via Caml Light, so when their designers rolled out OCaml, that was the only one I was following for a while until F# became a thing.

SML always seemed something that American universities were using. My only contact with it was reading the book "ML for the working programmer".

Back when I was in the university, our languages group department was very found of Caml Light, and used it for quite a few classes related to programming languages and compiler design.

DIKU Introduction to Programming organization on github has "ML Picture Library (for teaching purposes)" (https://github.com/DIKU-IP/InstagraML)

MoscowML credits some authors from Diku

http://studies.ku.dk/masters/it-and-cognition/profile-and-ca...

http://diku.dk/english/calendar2011/using_llvm_to_implement_...

https://en.wikipedia.org/wiki/Mads_Tofte

I cannot find a link to an SML course taught there but you'd expect one.

Also look at http://www.elsman.com/mlkit/ which incorporates many modern features, some of which can be found in similar form in Rust, recalling that the idea for regions wasn't invented by and is older than Rust.

There you'll find more Danish SML folks and links to SMLtoJs and somewhere down a link to the Smackage SML package manager and smbt Smackage build tool.

SML doesn't appear to be as dead as it seems from the outside.