Including a template with a variable in the href-attribut doesn't work in this case.

Including and importing other style sheets is a compile time feature.
Parameters and variables are bound at run time. This means: you can't
use a variable or parameter in the href of an import or include. The
value is used literally, it is never interpreted as XPath or AVT.

Bu what did I do wrong?

Not reading the spec? BTW this is a FAQ. The XSL FAQ should have an
entry for this.