Reading the Wikipedia article, it sure sounds like it was worth trying out once as an experiment. At the very least, it established that 50kW was a good maximum.
My guess would be public vs private. A lot, if not all, of the stations on that list seem to be government owned but in the USA they are corporate owned. 2000kW is fine if you own all the airwaves and don't have a company in the next city over complaining that you are crowding out their airwaves (of course they might have had other countries complaining...)
That's a story from the early history of radio. There were various proposals for how radio would grow. RCA wanted to have three high power stations to blanket the whole US. But a monopoly on radio didn't go over politically. Hence the 50KW power limit.
The power increase from 50 kW to 500 kW is only 10 dB, which isn't really a lot considering you get the same increase going from 5 kW to 50 kW, or from 500 watts to 5 kW. You can see that there's a dramatic point of diminishing returns for any given class of radio service.
International SW stations are just that -- international -- so they're more likely to be able to justify the additional power.
