Not many characters left in ASCII. Reuse of an existing operator almost required. Binary operators cannot be use as it would be grammatical ambiguous and would need resolution at the semantic pass which is a big no-no (that's why C++ is so slow at compiling, it cannot be parsed without semantic analysis). This left only the two exlusively unary operators !, ~. As ~ was repurposed for string concatenation, only ! remained.

    templ!thing(a,b)   

I would have thought that templ(thing)(a,b) would have been a good solution, as it is what is used in the declaration/definition side of templates, but this would have made removing redundant () not possible in UFCS expressions.