It’s the same reason why we have single-processor and multi-processor versions of, say, Linux kernels.   (And why those kernels also have tests for the presence of multiple cores, with instructions to halt every core but one.)   If you know that threading will never be used, mutual exclusion gates can be omitted from the core interpreter.   (And, AFAIK, any attempt to use threading will be detected and prevented.)   But would I bother to do it?   Probably not ...