Galileo's principle of relativity stated that it is impossible to perform any experiment within an inertial frame of reference (constant velocity, 0 acceleration) to detect the frame's motion. The only way to detect the motion of an inertial frame of reference is by referring to another frame of reference. Thus, if the aether theory was correct, measurements of the speed of light from an object moving with constant velocity would give varying values depending on which direction the object was moving relative to the aether allowing an observer to determine that they were in an inertial frame of reference. This violates the principle of relativity.
Einstein mused over this dilemma. In order for the principle of relativity to not be violated, then the speed of light must be constant in all frames of reference and the aether must not exist. However this would mean that distance and time are now relative -> the crux of special relativity.
Einstein's strong belief in the unity of physics led him to decide that the principle of relativity must not be violated leading to his development of his theory of special relativity in which he postulates 1. That the laws of physics are the same in all frames of reference (principle of relativity always holds) and 2. that the speed of light is constant regardless of the observer's frame of reference, and thus, distance and time are relative. Therefore, aether is non existent and its importance regarding the explanation of light's behaviour is redundant. Essentially, the violation of the principle of relativity did not sit well with Einstein and drove him to develop his special theory of relativity which experimental evidence actually supports, unlike the aether theory that violated Galileo's principle.
