People get mad that George RR Martin can't get past his 5th book in a decade. And here, Knuth has to spend like 40 years writing his 4th book (which has now expanded into 3 books unexpectedly: 4a, 4b and 4c). /s I joke of course. But TAOCP's future chapters are some of the most interesting computer challenges available. Every section Knuth writes is another treasure. It's clear that the computer science field really…
I wouldn't guess at or disparage Martin's motives, but with Knuth, one suspects that the motives are just what it says in the title: art, and computer science. Like Orson Wells pitching the vinter, Knuth will "sell no wine before its time". https://youtu.be/PUunRgUkRjQ
But yeah, Knuth's legitimate excuse is that he started writing the encyclopedia in the 1960s, when it was still conceivable that one multivolume work could cover all of the (at the time) small field of Computer Science.
But the 1970s and 1980s explosion made that fully impossible. We're lucky to get the chapters Knuth has written so far.