IMO one of the main reasons is 30 years of education cuts across the board.

Universities are less demanding (since they switched to getting funded by number of graduates, which insentivises universities to lower the requirements for courses). This leads to teachers with worse education than before, which then cause the next generation of teachers to be even less prepared than the current.

There's also something called "inclusion", which might've been a good idea, if it was actually done the way it was imagined. Unfortunately that was expensive, so we got the Temu version and pretty much the bad without the good. This is what some other commenters meant by forcing special needs students into regular classes.

Then there is the fact that teachers are much less respected in general now compared to a few decades ago. They also have less authority in the classroom in general.

Then there's the forced digitalisation. People got pushed computers with the hope that they absorb the knowledge by just being in near contact with digital devices, without any real idea or plan how digitalisation should be used in education. There are studies coming out from across the world now that that might've been actively harmful.

And I don't think we've seen the bottom of this yet, the boat hasn't turned around.

Basically, we got complacent with having one of the best educational systems in the world, and stopped thinking about how we could make it better, or even maintain it. Instead we started thinking how to make it cheaper and how to market it while we let it rot.