It's a lot of things I think.
A lot of parents feel they are being criticised if we tell them about their child's poor behaviour in school. Some of that is guilt because they feel responsible; some of it is anger that they feel blamed; some are just so disillusioned by the education system that they feel we should get our house in order before complaining to them about theirs without really understanding that their child's class teacher has very little autonomy or control.
MH services (adults and CAHMS), early help services and SEND provision have been slashed so parents who are struggling and might have got support 15/20 years ago feel they are being criticised but not helped. You only have to read posts on this and other threads to see the frustration some parents (justifiably) feel. In many cases, we're not telling them anything they don't already know but they feel angry and frustrated that we're dumping the problem back at their feet.
The rise of parent groups chats on WhatsApp and the like mean that, rather than dealing with grievances privately, they have a forum to air their discontent and other parents are not a voice of reason but an echo chamber of discontent.
A rise in individualistic thinking and language - eg triggering, validating, personal identity etc which remove the responsibility from a collective and social one to an individualistic one where no one can be challenged.
A lot of the parents of younger (primary) children today are the children who grew up within the non-competitive zeitgeist of championing everyone and not comparing (eg sports days, not displaying targets on the wall, not sharing with parents where children were in relation to other children but focusing on the progress they had made) and simply aren't used to being told they're not always right.
I haven't explained that last one well but one of my friends is a GP who said 10 years ago that a lot of the late teens/early 20s patients he was seeing were there for MH/anxiety. He said that, in the majority of cases, there was a sense that these were kids who had grown up believing they were perfect/brilliant/beautiful/clever/right etc because parents had told them this to boost their self esteem. The result was young adults who had little to no resilience and found it difficult to function in a world where the view they had been given of themselves by their parents etc wasn't shared by wider society. Many of those people will be late 20s/early 30s now so the parents of primary school children.
Both parents are often out of the house working long hours; an increased reliance on childcare and an accompanying lack of understanding of child development. Not criticising parents for that - I did it - but I also recognised my children's needs and so largely managed to offset the impact. Some parents don't have the knowledge to do that.
Lower grandparent involvement due to women's retirement age increasing and grandparents working longer. More focus on grandparents enjoying themselves than spending time with the family. Eg fewer grandparents who babysit or play an active role in their grandchildrens' upbringings because they've done their time childbearing and now it's time for them. Again, not criticising that in any way, but I do think it goes some way to explaining. I spent a lot of time with my grandma, who would play with me and take me on days out, babysit and I had a great relationship with her and her friends. My children didn't have any of that with their own grandparents and their lives were definitely poorer and less rounded because of it.
People having second families/blending families which aren't always welcomed or managed well and the resulting impact of those. And even when they are managed well, there is often a child somewhere who feels sidelined and invisible.
Not all of what I've mentioned is problematic in and of itself, but the combination of many of these factors has contributed to the shift. I believe.