Math typesetting was a goal, but it wasn’t really the main goal.
Knuth started TeX because the proofs for TAOCP Volume II were done with early phototypesetting equipment, and the results were terrible. Not just terrible for math, but very poor even for just regular prose. The reduction in quality from the Linotype machines used for Volume I was unacceptable to him, but it would have been far too expensive to revive that obsolete typesetting process for one book, so he decided to write TeX instead. His goal here was to knock the whole typesetting process so far out of the park that he wouldn’t have to deal with this same problem again for Volume III, Volume IVa, Volume IVb, Volume Va, etc, etc.
Knuth started TeX because the proofs for TAOCP Volume II were done with early phototypesetting equipment, and the results were terrible. Not just terrible for math, but very poor even for just regular prose. The reduction in quality from the Linotype machines used for Volume I was unacceptable to him, but it would have been far too expensive to revive that obsolete typesetting process for one book, so he decided to write TeX instead. His goal here was to knock the whole typesetting process so far out of the park that he wouldn’t have to deal with this same problem again for Volume III, Volume IVa, Volume IVb, Volume Va, etc, etc.