As always, there's no single issue.
Are the issues really that much worse or are they a bit worse, but seem worse because a) we have more children (DS' school, for example, had just 6 form entry a few years ago, now it's 8 form entry - that's a lot more children so if the numbers are up, are the ratios?). Also, similarly, society has changed a lot and a lot of children who would have struggled at school might not even have been in school, and certainly weren't always as visible and tagged for support, which again, messes with ratios.
Then there's Covid - the gift that never stops.
Then there's the fact that the vast bulk of schools have improved in terms of their thinking on behaviour and other issues. Punishments and codes still exist, but increasingly schools are expected to look more deeply etc.
In the last few years we have the COL crisis and the adult burn out someone else has mentioned. This can make things tougher for families, and more likely that families are struggling with the basics, never mind the more complex issues.
Then there's an increased mindset of paranoia. It wasn't that long ago that children were routinely allowed or even encouraged to play out, walks to school etc from a relatively young age. Studies have shown this helps children build resilience and problem solve. But now, even when parents want to allow this freedom, it can be difficult - eg our school will not allow children to walk alone pre year 6. Plus there's the censure you get from the broader community...
These are just the issues I can think of off the top of my head. I'm sure there are a lot more.