Long story short...

1) The United States has the largest aviation industry.
2) Flight training here is much cheaper
3) There has never been protection for American jobs no matter what industry you've looked at
4) Other countries want to grow their aviation industry and want to hire from within
5) Other countries are not (were not) as financially advanced as the US and thus need to keep those jobs to boost their economies.
6) Those countries are allowed to discriminate based on all things we are not.