Granted they had some bad keyboards; there's a oft-reproduced photo of Thompson & Ritchie at Teletype 33s, but those had '63 ASCII, including ←. Probably more

Granted they had some bad keyboards; there's a oft-reproduced photo of Thompson & Ritchie at Teletype 33s, but those had '63 ASCII, including ←. Probably more important is that the Unix/C developers came from Multics, written in PL/I, which (following Fortran) used ‘=’ for assignment. And Fortran was still important; Unix had a Fortran compiler at least as far back as Second Edition, when C was being born. Kernighan & Plauger's Elements of Programming Style used Fortran and PL/I for its examples, and Software Tools used Ratfor. ‘=’ is simply what they were used to.