Compare, so called "ICAO" standards throughout the world and therein lies your answer.
The facts are that standards still differ greatly from country to country and, for example, whatever the EEC may dictate regarding freedom of the individual to work in any other EEC Country excercising their professional qualifications, the standards of some countries just don't match up to others.
How many non UK nationals, who have qualified as ATCOs in other countries, have managed to meet UK CAA standards and get jobs here? Very few is the answer.
Its not a case of our profession being "International", it's meeting the highest standards and making sure everyone complies with that at every level, all the time.