IMO besides being extremely easy to program, they didn't required extensive tooling to make interesting programs specially when you compare with alternatives at the time.
Another interesting aspect was the "statelessness" of the system. You had one disk with your programs, other with data and so on, a crash wound't affect or make you lose everything since you're a reset away from coming back to where you where
Even the simplest programming solution today is way more complicated than what existed back them. I wish we had more boot to work systems to hand to kids without much interruption.
My kids are getting into the age of programming and understanding how hardware and a computer works is way more easy when you don't have tons of abstractions on top of it. That's the reason I've been following the David Murray's commanderX16 initiative anxiously :)