I'm not sure what these numbers are measuring, but it looks like less than 85% of schools in the US reported their data, could be a sampling bias there. Also, this is maybe at age 15, when many of the worst readers have already dropped out of school.
It looks like most countries have shitty literary levels, then. I was unaware other countries were just as bad. A quick Google showed a few results not quite in agreement saying the average for adults was 7th-8th grade in the US, or that half were below 6th.
I think the gutting of school funding, and the adoption of an idiotic non phonics based reading curriculum that was sold basically by some grifter over the last few decades is more to blame. Also Bush's ~~no child left behind~~ Pass All the The Failing Kids Anyway plan.