"who had a computer at home (with a modem)"
The problem was the modem - many computer didn't come with one, and they were expensive. Even in most companies you didn't have one. Also, in many places outside larger cities without a local number to call, accessing Usenet, BBS or the like was expensive. Moreover you kept a telephone line busy.
"vi was built by programmers, for programming"
That was true for TurboPascal as well - but TurboPascal was built around the IDE idea. in DOS being able to edit, compile and debug from the same applications was very welcome, since you had no multiprocessing. Tools that required to edit, exit, compile, return to the editor to fix errors, rinse and repeat, run, debug, return to the editor, etc. etc. were far less friendly. That was one of the reasons of TP success.
Moreover many DOS programmers didn't have a Unix background, a lot learned programming on 8 bit computers. and after all, vi looks too much like edlin... <G>