Outside of environmental factors, social factors should also be considered.

Oversimplified: Maybe everyone is being too safe and not exposing children to stimuli that promote better development.

I suspect several factors are contributing to this. 1) As more toys are discovered to be 'dangerous' or 'have small parts' children have less exposure to complex objects or it is deferred to later ages. 2) Adults have less, or are less inclined to, time to properly care for children and thus don't understand that judgement and supervision can allow 'supervised' use of toys that can be dangerous if not properly used, but would be fine if used while supervised. 3) I wonder how strongly this correlates to all parents within child rearing units being in the workforce (single parents, dual income parents, etc)? 4) Or how it relates to decreasing access to natural areas to explore as a child?

These are only some of the social influences that could also be part of the cause.