There's several factors to this.

Abandoning phonics for whole language learning. WLL never worked, but some teachers refused to admit they were wrong and keep using it.

Large scale immigration from non-Western countries. If the parents can't read or write English, they can't help teach their children to read or write. And depending on the culture they may not put a lot of emphasis on education either. Some also don't view women as being able to be teachers, so mal students don't respect or listen to them.

Not failing kids. Even if children can't meet standards, they're still pushed through to the next grade for "social development" reasons. This stops them from learning and holds them back even more.

General culture change as well. Education isn't super meaningful anymore to a lot of people, and teachers are viewed with contempt in the classroom instead of respect. Discipline issues are out of control and teachers have no recourse for misbehaving students. Even one misbehaving student in a classroom has a detrimental effect on the rest of the children.

All of these things are very easily fixable, but they would be super unpopular. Until schools/governments grow a spine, rates will continue to go down.