[EternalVigilance]

The reason I know is that light has never been observed to go faster than C. There is no reason to believe that light was faster for some time before it slowed down to its steady current speed.

If you see a car going 50 miles an hour, do you assume it has always traveled at that speed?

[albionin]

Apples and oranges. Cars can travel at any speed up to a certain maximum withing the parameters of the motor. Light however does not. It can slow down slightly as it moves through glass or water, but it cannot travel faster than C. If cars were known to travel at a constant speed such as C and so reliably so that we could use that speed as a constant in mathematical equations then there would be no reason to expect the car to change speeds when we are not looking. And neither is there a reason to believe that light is breaking the laws of physics when we are not looking.