I am aware of the basic history of TeX and LaTeX and Dr. Knuth's motivation for creating TeX. Mainly, he was disappointed with the quality of the galley proofs of the second edition of his book after the monotype system was replaced by phototypesetting one. That occurred in 1977. Thereafter, he started to work on TeX and officially released it in 1978.
I will make an assumption that creating TeX is a project of decent magnitude. Wasn't there any other solution other than creating TeX? By that time, Bell Labs have already developed troff (1973) and eqn (1974) for Unix which, if I am correct, was starting to be licensed to academic institutions during that time (late 70s). Was Donald Knuth aware of those or had access to them? I am also wondering whether there were other similar alternatives.
I know that those text processing programs are inferior to TeX, but I am sure that if they were available to him, they should have done a good job providing him great tools for writing a book. Thus, my question boils down to this: Were there other programs available to him? If yes, what made him create TeX? Did he find them insufficient or just wanted to make his own program?