I remember what setting the time on a VCR was like and it's interesting to think of all the assumed knowledge you actually need in order to have it seem intuitive.
Two things off the top of my head that I can think of: 1) knowing that a blinking number is indicating some kind of selection and more generally 2) seeing the UI as a glimpse into a larger abstract space that can be navigated. Or in other words, having used computers for many years, what my parents sawzl as just a blinking word, I would see as a menu where up/down/left/right had meaning.
There's also some more abstract thinking involved there - for me it's very spatial so I think of it as being able to keep track of your place in this 'abstract map'. You had to learn some non-obvious things like "if the down button stops working, it probably means I'm at the 'bottom' of my available choices" or "if I start seeing the same choices again, it means I have 'wrapped around' and in a logical sense I'm back to where I've been before".
I actually remember thinking something like this as a child when we got a VCR. I think I remember that realization that "this is a menu I can explore". The exploratory skills you pick up when you have to figure out how to use something technical generalize really well to other technical things.
TL;DR: I think VCRs were hard to program because the limited UI of buttons and a tiny screen meant that you actually needed a fairly built-up mental model of the process to keep track of what you were doing.