I think Borland Pascal only became popular because the PC at the time was so extremely limited in memory and speed, so that a compiler for a simpler language made sense.
Borland Pascal wasn't really meaningfully simpler than Modula-2, though. It had modules (units) with separate compilation, and all kinds of low-level primitive, down to inline assembly. At some point (IIRC it was version 5.5? either way, still late 80s) it even became a full-fledged object-oriented language. In terms of what you could do with it, it was definitely comparable with C and C++ compilers available for DOS at the time, and separate compilation helped compile speeds - the short compile time of Pascal, and later Delphi, was truly legendary. They also had what was by far the best DOS IDE, with syntax highlighting, integrated debugger and help system etc. Granted, this was also true for Borland C++, but that was more expensive.
Yeah, on Unix, it never really got off the ground because C was the system language there. On DOS, it was a whole different world.