When you step outside your familiar community, you learn interesting things about your assumptions. Something I've learned on the ECMA committee is that in Scheme, new syntax is "cheap," but not so in other languages.
Because syntax definition is in the hands of Scheme user programmers, we expect to see new syntactic forms all the time. As a result, implementing a feature as a syntactic form is uncontroversial in Scheme. In other languages, however, a new syntactic form signals (at least intuitively) a global change to the language. Users know that a new API is something they can put off learning until they feel they need it. But if there's a syntactic form in a language that you don't understand, you feel as though you don't have a handle on the language.
In terms of pragmatics, it's easier for language implementors to implement a subset of an API than it is to implement a subset of the syntax; you can at least guarantee that all programs will parse the same. What this really comes down to is scoped syntactic extensions, i.e., macros. If you can modularize syntactic forms, you can signal a parse error that says "I'm sorry, I don't support this syntactic form," which is the same as "I don't support this API."
In terms of cognitive load, modularizing syntactic forms allows users to learn subsets of a language in exactly the same way they learn subsets of libraries.