Unfortunately, the speed of light kept changing when they measured it. It was different at different times of day.
That's not surprising because the speed of light is only fixed for the perfect vacuum and without any gravitational influence.
But in this universe there is neither a place where you can find a perfect vacuum (The Heisenberg's uncertainty principle sees to that) nor a place without any gravitational influence.
And it's not just the speed of light. It's the speed of everything without mass. Light was just the place we noticed it first. And in 4D space time it's also the speed of everything with mass.