A doubt about the classic problem of measuring the speed of light in only one direction

Why can’t two clocks synchronized with signals SLOWER than the speed of light, measure the speed of light in one direction?

I know that it is not possible to measure the speed of light in only one direction, and I understand why two clocks synchronized with light signals are useless. But I realize that I can’t explain why two clocks synchronized with signals slower than light don’t work either.