To add to the "simplicity" aspect, I'd add: They were tiny.

You could read the entire OS ("KERNAL") and BASIC disassembly start to finish (there were books listing them, with comments added). You could systematically test what changing registers would do - I remember pestering my parents at work by calling them to let them hear what sounds I managed to make by randomly POKE'ing things into the sound registers just to experiment.

And of course the manuals. While I agree with you books were hard to get, the C64 manual was fantastic.