Relatedly, my favorite example of "why didn't this get invented before" is wheeled luggage which (from what I've read) wasn't sold until 1989, even though the wheelbarrow (the same basic idea) has been around since prehistoric times.
IIRC, the reason it didn't appear sooner was a combination of a) getting reliable castors[1] for wheels that small is a non-trivial problem, and b) there wasn't much demand for it until the 80s.
IIRC, the reason it didn't appear sooner was a combination of a) getting reliable castors[1] for wheels that small is a non-trivial problem, and b) there wasn't much demand for it until the 80s.
[1] These things: https://en.wikipedia.org/wiki/Caster