Because for the longest time there was nothing both better and standardized. I've dealt with xml many times and hate it but It's better than maintaining your own proprietary DTL.
Saying there are no s-expression parsers for non-Lisp is ridiculous.
This one's http://sexpr.sourceforge.net/ for C, it's been in sourceforge since 2002.
perhaps you prefer one in :
Javascript - http://planet.plt-scheme.org/package-source/dherman/javascri...
Perl - http://search.cpan.org/~nelhage/Data-SExpression-0.34/lib/Da...
Limbo - http://man.cat-v.org/inferno/6/sexprs
I shan't go on
But you'll note the disparity in support. A library for parsing XML with C may have existed since 2002, but it's existed for XML since 97. In Java, XML support has been baked into the JVM for a while, but S-exp parsers are hard to find. That library you list for Javascript is not a s-exp parser but rather a Scheme library for generating Javascript from a Scheme-like language. Googling for javascript and s-exps returns a lot of results like that but not many s-expression parsers. There certainly aren't any widely used and recognized libraries like there are for XML and JSON fro most languages aside from Lisp.