Market failures of the other Unices aren't necessarily related to the technical advantages or disadvantages or symbiosis with C or being implemented in C. However, making C programmers' life easier was crucial.
Linux was at the correct place at the correct time. It was the only free version of Unix-like OSes that didn't have legal bullshit to deal with. IBM and Intel's support also made GNU/Linux ecosystem successful, without them it would stay as an academic project. Being free meant that it had an advantage where price sensitivity mattered and dotcom boom and VC explosion is very sensitive to cheaping out and preffers suffering with less-than-ideal software. So Linux stayed popular while other ones died slowly.
C had a huge following and all OSes had to support it. Simplicity made it popular when average hardware at the hands of many academics and young professionals was very weak. Being written in C may have made things marginally easier but neglecting it for Ada or Pascal was a terminal mistake. Windows isn't Unix at all but it also had to support C well.
Free beer OS with source tapes and the Lions book made the huge following of academics and young professionals.
Had AT&T been able to sell UNIX, and naturally C, at the same price points as VMS, System 360, and many other contemporary OSes, and none of us would be talking about them today, other than history curiosities.
Instead we are left with UNIX haters handbook, and still trying to fix the security issues across the industry caused by C's adoption, the JavaScript and PHP of systems programming languages, both in adoption scale, and code quality.
Linux was at the correct place at the correct time. It was the only free version of Unix-like OSes that didn't have legal bullshit to deal with. IBM and Intel's support also made GNU/Linux ecosystem successful, without them it would stay as an academic project. Being free meant that it had an advantage where price sensitivity mattered and dotcom boom and VC explosion is very sensitive to cheaping out and preffers suffering with less-than-ideal software. So Linux stayed popular while other ones died slowly.
C had a huge following and all OSes had to support it. Simplicity made it popular when average hardware at the hands of many academics and young professionals was very weak. Being written in C may have made things marginally easier but neglecting it for Ada or Pascal was a terminal mistake. Windows isn't Unix at all but it also had to support C well.