[masor]

Aviation. 500 & 1000 ft intervals are very convenient discrete altitude levels for coordinating traffic, and that's probably a key reason why it's held up universally. In meters, the interval will either awkward and hard to communicate numbers or spaced too widely to be efficient.

[dorfsmay]

I think the reasons are historical, because the U.S. pretty much invented the industry.

I believe the eastern block used to use meters, I doubt 300m was much more difficult to deal with mentally than 1000 feet!