Looking at the amount produced by year it seems like U.S. production has been relatively steady over the last year and not too much lower than it was in previous decades. I think looking at percentages doesn't really tell the full story here, since China's production has increased so much as to overshadow most other countries' output. The "drop" from 25% to 5% wasn't as much due to production leaving the US as it is production not increasing at breakneck pace.