Because the benefit was probably seen as very little, and the cost significant. When you're writing a compiler for an architecture where every byte counts you

Because the benefit was probably seen as very little, and the cost significant. When you're writing a compiler for an architecture where every byte counts you don't make it write extra code for little benefit. Programmers were routinely counting bytes (both in code size and data) when writing Assembly code back then, and I mean that literally. Some of that carried into higher-level languages, and rightly so.