In the US one of the big problems is a culture of legal liability for every possible thing, and a general culture of fearfulness, excuse-making, and ass-covering instead of allowing moderate informed risks and dealing responsibly with the consequences. If you have too steep a slide, some kid is going to fall off and the parents will sue and bankrupt your town. If you let kids ride the bus by themselves, 1 kid in a million is going to get kidnapped and the parents will sue. Etc.
But there are surely other contributing factors: more families with 2 working parents (and more single parents) and in general less adults “hanging out” with an eye on their neighborhoods, a general degradation of community relationships and civic institutions, smaller family sizes (it’s much easier to chaperone 1 kid than 6), more middle-class angst about maximizing children’s future earning potential, more media attention on rare tragedies, communication improvements leading to less spontaneous social gatherings and more virtual socialization, increasing reliance on car transport and inaccessibility of unsupervised play spaces (especially undeveloped land), etc.
