There are three major implementations polyML, SML/NJ, and Mlton (the last two are used together a lot as Mlton's whole code optimizer can take some time). Most of those coders work at universities and the biggest projects (for polyML at least are large theorem provers). They don't really focus on the software problems of more typical businesses.

There are very few languages that rise to prominence without corporate intervention. SML is a solid foundation, but the ecosystem is somewhat lacking. I don't really know aside from that. Even though SML syntax isn't difficult or particularly radical, it isn't in the C family which (I believe) makes it a no go for lots of companies.

EDIT: to answer more clearly, we simply need more dev time to create and improve the library situation and that basically demands a corporate patron.