I'd say it is because Turbo Pascal and then Borland Pascal got popular enough, and had TurboVision and other stuff.
Modula-2 was a nice language (and modern Go is built basically on its ideas, plus Oberon's), but it had a hard time competing for the same niche on the PC. It worked better in embedded space, though.
On Unix, there was C, a language with many shortcomings, but one big upside: it was the language the kernel and the userland was implemented in. Its compiler was readily available as a part if any Unix installation. It seemed a natural choice over Pascal, unless you wanted drastically different features (then you had Fortran, or awk, or Tcl, or later Perl, etc.)
Modula-2 was a nice language (and modern Go is built basically on its ideas, plus Oberon's), but it had a hard time competing for the same niche on the PC. It worked better in embedded space, though.
On Unix, there was C, a language with many shortcomings, but one big upside: it was the language the kernel and the userland was implemented in. Its compiler was readily available as a part if any Unix installation. It seemed a natural choice over Pascal, unless you wanted drastically different features (then you had Fortran, or awk, or Tcl, or later Perl, etc.)