ATAthas·Jan 1, 2017
I work with a bunch of people who were very involved in SML, and I've asked this question many times. They usually say that SML (as a language) stagnated because of personal conflicts in the standardisation committee. A bit of a "worse is better" microcosmos, really, with a focus on on discussing theoretical perfection instead of how to get things into the hands of users. You can partially see this in play today in the Successor-ML[0] discussions, where there's not really any clear vision or leadership driving things forward, just a lot of (often very good) language suggestions.
My own extrapolation is that a multitude of SML implementations meant that programmers couldn't take advantage of implementation-specific extensions, as that would prevent their code from running elsewhere. This hindered real-world experimentation (as opposed
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to academic investigations) and organic growth of the language.
But, I should say that as starting point for creating new functional languages, SML is an exceedingly clean design. I'm using it myself for my own language design efforts.
[0]: https://github.com/SMLFamily/Successor-ML/issues