Having loved Pascal and struggled with Modula 2, I'll go out on a limb and I'll say: for cosmetic and typing issues.

Modula 2, at least in the implementation I had to use at my University, had case-sensitive keywords which were in UPPERCASE. Compared to Pascal's case-insensitive keywords, in the editors available at that time, Modula 2 stuck out like a sore thumb and made writing and reading code like a chore.

When I started grad school, in 1986, I'd written a lot of Pascal and used Turbo Pascal on my PC. But the school used Sun workstations with unix, so I had to learn C pretty quickly. Nobody seemed to use Pascal. There was a Modula II compiler around, but it was a resource hog and never popular. Oberon was published about the same time. I thought it was pretty cool and worked on a compiler for it, though the language evolved out from under me. My impression is that none of the Wirth languages made much of an impression in any of the US grad schools in that era. Of course, I didn't see everything that was going on, but I saw most of the compiler work. Almost 100% in C, moving eventually to C++.

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.)

Because Turbo Pascal already had modules.

I remember the Modula-2 I used in 1988-1991 was a royal pain in the butt to do any I/O. It was a pain compared to Turbo Pascal and, as Koshkin stated, already had modules with a significant ecosystem.