The following messages, from the Lucid Emacs mailing lists in 1992 and 1993, comprise the bulk (if not the entirety) of the public discussions between the Lucid and FSF camps on why the split happened and why a merger never did.
For me, this is like finding a cache of grandpa’s old letters in the attic.
The Feb 1993 messages about using an X toolkit or not, etc.. Wow! So that’s why emacs is how it is!! At least as recently as 24.x, you have to pick a toolkit at compile time… This argument was … codified!
One quite interesting part for me was Marc Andreessen’s involvement, particularly in light of his and JWZ’s future at Netscape, etc.