This article by Abel Avram The Scheme Language Is to Be Split in Two caught my eye. Basically scripting needs a small language, while application programming needs a big language.
Now it is not of course the first time a standard has been split. XML is an example.And time will tell whether it is a fork or a layering/subset arrangement.
But I like the basic idea, that when there is a long-time standard with no shortage of goodwill, but we see a proliferation of dialects, then there is something wrong with the standard.
I often defend SGML (especially when it used as a bogeyman) but I certainly think that HTML represents a dialect that SGML should have been able to respond to. (In XML, ISO at least got it right and altered SGML to cope, which I suppose is because the XML bothered to talk to the SGML people as if they were colleagues not fossils and hicks.)
Mind you, I expect that there is cat-herding aspect: the people who write LISP systems are excited about features and exploration, otherwise they would presumably re-use an existing implementation. Nevertheless, for the people who say "XML is just S-expressions" we have the rejoinder, "Except that one XML system always recognizes the syntax of the XML from another system!" The fixity of syntax is key to XML but peripheral to S-expressions.