alt Hacker NewsCoalton is also a full-featured embedded language of Lisp that is sufficiently paradigmatically different than run-of-the-mill Lisp code you'd see in a Common Lisp textbook, since it has a complete, Haskell-like static type system and Lisp-1 naming. Coalton also sees active development because aspects of the language continue to evolve.
The consequence is that an integration with SLIME would have to be a very extensive contrib [1] that is shipped with the Coalton version the user is using, and updated whenever Coalton is updated. No doubt the contrib would have to be very elaborate—it would have to hook in to basically every aspect of SLIME and SWANK if it should be "Coalton-native", from the display of type errors to how auto-complete is handled. Unless the contrib author is very meticulous about backward compatibility, then version mismatches would make everyone involved unhappy. The contrib author would get annoyed at constant bug reports about things not working (even if there's a nice "your Coalton or contrib are out-of-date" error), and users would get annoyed they have to keep a Lisp library in sync with an Emacs add-on.
None of this gets to the matter that Emacs simply isn't a popular text editor, and it's not really the one people are rushing to learn, even if it has substantial merit. I don't know how trustworthy this source is [2], but it claims that Emacs represents a fraction of a percent of the developer community. Even if it's off by 10x, it's still 1-in-50 developers at best.
[1] There's a basic one that shows Coalton type hints, but not much more: https://github.com/slime/slime/blob/master/contrib/slime-coa...