Comment by pm215

It was simple specifically for the people writing it at the time. We know this, because they've helpfully told us so :-) It might or might not have been harder than a different approach for some other programmers writing some other OS running on different hardware, but the accidents of history mean we got the APIs designed by Thompson, Ritchie, et al, and so we get what they personally found easy for their PDP7/PDP11 OS...