When you avoid a breaking change in the early evolution of a language…
There were several hundred kilobytes of existing C source code in the world at the time. SEVERAL HUNDRED KB . What if you made this change to the compiler, and made an existing program wrong? That’s a potentially disastrous breaking change.
You might say “just search all the source code for that pattern” but this was two years before grep was invented! It was as primitive as can be.
It’s fifty years since this mistake was made, and since it has become embedded in popular successor languages we’ll be dealing with its repercussions for fifty more at least, I’d wager.