We ran out of people buying from parts stores - hobby electronics became less popular.
Eras of hobbies:
Mechanical
Chemical
Electronic
Computer hardware
Computer software
[What's now?]Chemical: I was thinking chemistry sets and the age when Chemistry was geeky cool (Du Pont, Uncle Tungsten).
My grand father was a structural engineer, my father studied chemical engineering, I studied electronic engineering (but got a job programming). So perhaps I was thinking more of the frontier of engineering shifting rather than technical hobbies.
It would be interesting to look at geeky hobby adverts in a magazine over time and see how the advert focus shifted.
> I was thinking chemistry sets
On the other hand, most mechanical things were relatively cheap. Cars were worked on since cars existed. People also worked on everything in their homes. There was the tradesman who takes a specific skill like being able to fix a washing machine, but a lot of these people had hobbies of building and making.
So we have a direct connection to the death of the repairman and death of the handyman. These were quite popular things even up through the 90's. You'll even see this in shows and movies.
Plus, you forgot the most popular hobby of them all: woodworking. Still alive, but nowhere near as popular as a few decades ago.
I don't think it has become harder to repair cars though. Most problems that need repair are the same old bushings, brakes, spindles, rust, bearings ...
I think it is some cultural change away from handy work in general.
Electronics have become harder to repair with smaller and more integrated circuits though.
Circle of life. ;-)