ETHZ OS were done in Modula-2, Oberon, Active Oberon.
Olivetti and DEC were using Modula-3.
Several OS were being done in Algol and PL/I dialects back when UNIX was being born.
C's ubiquity is a consequence of UNIX adoption by some successful startups in the workstation market.
If one of the other systems had enjoyed a similar adoption another safer systems programming language would have taken C's adoption.
ETHZ OS were done in Modula-2, Oberon, Active Oberon.
Olivetti and DEC were using Modula-3.
Several OS were being done in Algol and PL/I dialects back when UNIX was being born.
C's ubiquity is a consequence of UNIX adoption by some successful startups in the workstation market.
If one of the other systems had enjoyed a similar adoption another safer systems programming language would have taken C's adoption.