Another reason you won't find people using string templates to produce HTML in Lisp is that no one uses it for web development. This phenomenon where multiple language features conspire to prevent misuse is called defense in depth and is one of the great strengths of the language.
Ironically, the dominant solution for dynamic HTML in Clojure apps, Hiccup, does not rely on macros as much as it relies on keyword and collection literals.
Reddit apparently ran away from Lisp primarily because all the servers ran on FreeBSD while development ran on Macs and because at the time they were forced to use different Common Lisp implementations (OpenMCL on Mac and CMUCL on FreeBSD) so they couldn't even test what they were deploying, essentially. Today with SBCL that wouldn't have been an issue.
> Another reason you won't find people using string templates to produce HTML in Lisp is that no one uses it for web development
...how does this change conditional probability? If of those people who use Lisp for web development, nobody uses strings, it's unrelated to how many people use Lisp for web development.
> This phenomenon where multiple language features conspire to prevent ~~misuse~~ use is called defense in depth and is one of the great strengths of the language.
Not only do they use it for web development, but they manage to regularly update and upgrade their Lisp based web apps (as opposed to ignoring customer emails because their pile of PHP/Perl is too hairy to debug).