Short answer, UNIX and politics.

Long answer is a bit more complex.

UNIX was the only OS where C was relevant, and AT&T was forbidden to sell it, so they made the source code available to universities and companies for a nominal price.

That was already a big difference, instead of paying a sum of several thousands for closed source OS stuck to a specific mainframe, companies and universities could get something with source code for about $70 (if I remember correctly).

Thus UNIX eventually became the foundation of 80's startups like Sun and SGI.

Home computers firmware was basically Assembly, where C (actually K&R C dialects) was yet another language among all the available ones.

When the 16 bit revolution happened, there were several 16 bit computers systems done in a mix of Pascal and Assembly. The most notable ones Apple Lisa, Mac OS, MicroEngine and Corvus Concept.

http://basalgangster.macgui.com/RetroMacComputing/The_Long_V...

http://basalgangster.macgui.com/RetroMacComputing/The_Long_V...

https://en.wikipedia.org/wiki/Pascal_MicroEngine

https://en.wikipedia.org/wiki/Corvus_Systems

As a notable example, Photoshop started as an Object Pascal application, which was later on ported to C++.

http://basalgangster.macgui.com/RetroMacComputing/The_Long_V...

So you had UNIX variants gaining a foothold on university campus and companies, with people wanting to write applications on their tiny (cheaper) computers at home, which like JavaScript and the Browser nowadays, C eventually escaped UNIX.

Apple eventually gave in to market pressure and rewrote their Object Pascal based SDK into C++ and C. And even tried their first stint at UNIX with A/UX.

http://basalgangster.macgui.com/RetroMacComputing/The_Long_V...

Corvus Systems tried to pivot their computers into Concept Unix workstation.

The Amiga OS was written in a mix of Assembly and BCPL, and eventually got its UNIX port.

https://en.wikipedia.org/wiki/Amiga_Unix

C by virtue of being UNIX's system language meant that its tooling was free with the OS, while any other programming language had to be bough separately and still you would need to write FFI to C at some level.

As side note, eventually C compilers stopped being available for free when Sun started the trend of selling the SDK, which made the largely ignored gcc to start getting contributions.

While this was happening, Brian Kernighan decided to push for C, with articles like

"Why Pascal is Not My Favorite Programming Language"

http://www.lysator.liu.se/c/bwk-on-pascal.html

Which ignored the fact that the majority of Pascal dialects had features that covered his complaints, specially the second ISO revision, Extended Pascal. And that outside of UNIX most C compilers were actually dialects, which is one of the causes for UB.

There were other bash articles flying around like "Real Programmers Don't Use PASCAL".

http://www.ee.ryerson.ca/~elf/hack/realmen.html

On the PC world, OS/2 and Windows were being written in C, but the majority of application developers were actually already jumping with C++ wit C Set++ for OS/2, OWL, VCL and MFC on Windows.

Another nascent PC OS, BeOS, was also being written in C++.

It was the rise of FOSS UNIX software, with C as its systems language, that shifted the balance towards C. To stress it again, just like JavaScript on the browser.

If you look at the history of systems programming languages, the ones that lasted longer were always the ones that were the way to write software for a specific OS.

Regarding Pascal, the majority of vendors started to consider Turbo Pascal as the pseudo-official standard, which when Borland decided to ignore small developers and focus on the enterprise, just kind of killed the Pascal market. Specially since Turbo Pascal was only focused on PC.

There is probably a bit more to rant about, but I guess you get the idea.