it's not like we didn't know about safety back then, but computers were slow, memory was limited, compilers had to be small, optimization was rare, and C was screamingly fast.
At the same time, when major compiler vendors couldn't even get their Ada compilers out the door in project overruns measured in years, Mitch Kapor's 1-2-3 was written and took over the spreadsheet market (dethroning the incumbent Visicalc) and it was written in assembler, not even C, and barely had memory to spare on user machines (memory? there was barely address space left to spare). The tradeoffs were authentic, and there was zero enthusiasm for Ada among programmers or entrepreneurs looking to get products launched. The mainframe world could have been different but the personal computer software market was exploding in a way larger systems never did.
(oh and let me add, all the undefined behaviors back then were very clearly defined (perhaps over a small field of options) and extremely useful, and should have been left that way; shoot out a compiler warning or error if they get your panties in a bunch)