Why does a Turing machine recognise exactly one language?
The language recognized by a Turing machine is, by definition, the set of strings it accepts. When an input is given to the machine, it is either accepted or not. Any particular input to that machine is either always accepted (in the language) or always not accepted (not in the language). So there's no mechanism by which a single Turing machine even could accept more than one langauge.