I suspect in a lot of cases reading the Lions commentaries was not purely for fun. The original audience (Lions' students) were doing it as part of their university course. Many of the contemporary readers would have been reading it because they needed to understand, fix bugs or add features to their unix systems (I think one of the introductions in the reprint book is from somebody who was in this position). But you're right I'm sure that the the key was the combination of (a) real world and genuinely useful code that the readers were using (b) size such that you could realistically understand all of it (c) excellent accompanying code commentary.

I read through the Lions book in the late 1990s when the reprint came out, essentially "just for fun" -- it remains probably the only sizeable piece of code I've read for fun.