Because almost nobody had a computer that supported it when C was created. That article mentions 3 cpu architectures that have the instruction in the 1970s:

Because almost nobody had a computer that supported it when C was created. That article mentions 3 cpu architectures that have the instruction in the 1970s: 1961: IBM Stretch 1964: CDC 6000 1975: Cray-1 After that, it mentions further support in 2005, with Sparc. Reading https://en.wikipedia.org/wiki/IBM_7030_Stretch https://en.wikipedia.org/wiki/IBM_7030_Stretch, a grand total of 9 IBM Stretch’s were sold. In comparison, the CDC 600 was a wild success, with “100+” sales (https://en.wikipedia.org/wiki/CDC_6600 https://en.wikipedia.org/wiki/CDC_6600) The Cray-1 scores similarly with “over 100” (https://en.wikipedia.org/wiki/Cray-1 https://en.wikipedia.org/wiki/Cray-1) C is from 1972 and started life for the PDP-7 and was soon ported to the PDP-11, of which “around 600,000” were sold (https://en.wikipedia.org/wiki/PDP-11 https://en.wikipedia.org/wiki/PDP-11; I guess most of these will have been sold late in the 1970s and early 1980s, but I think it’s a safe bet they were almost immediately more common than those 3 machines)