The principle of relativity and Einstein's constant-speed-of-light postulate are incompatible because the former is true and the latter is false. Truth and falsehood cannot be compatible. And the falsehood would have been exposed long ago if Einsteinians had not adopted the ether theory's wrong idea that the wavelength of the emitted light varies with the speed of the light source:
Albert Einstein Institute: "We will start with a very simple set-up, which you can see in the following animation. On the right-hand side, drawn in green, there is a sender that emits pulses in regular succession. On the left-hand side there is a receiver, drawn in blue. The pulses themselves are drawn in red, and they all travel at the same speed from right to left. Everytime the sender emits a new pulse, a yellow indicator light flashes once. Likewise, a flashing light indicates when a pulse has reached the receiver:
Next, let us look at a slightly different situation, where the source is moving towards the detector. We assume that the motion of the sender does not influence the speed at which the pulses travel, and that the pulses are sent with the same frequency as before. Still, as we can see in the following animation, the motion influences the pulse pattern:
The distance between successive pulses is now smaller than when both sender and receiver were at rest. Consequently, the pulses arrive at the receiver in quicker succession. If we compare the rates at which the indicator lights at the receiver and at the sender are flashing, we find that the indicator light at the receiver is flashing faster." [END OF QUOTATION] http://www.einstein-online.info/spotlights/doppler
Einsteinians make the following assumption above, which is essentially identical to Einstein's 1905 constant-speed-of-light postulate:
Assumption 1: "The motion of the sender does not influence the speed at which the pulses travel."
Assumption 1, a tenet of the ether theory, goes hand in hand with another assumption, also a tenet of the ether theory:
Assumption 2: "The distance between successive pulses is now smaller than when both sender and receiver were at rest."
Assumption 2 is false - the pulses do not bunch up when the source (sender) is moving. If they did, by measuring the (variable) distance between the pulses, an observer associated with the source would know whether he is moving or at rest, which contradicts the principle of relativity.
Since Assumption 2 is false, Assumption 1 is false as well. If the speed of the moving source is v, the speed of the light relative to the receiver is c'=c+v, in violation of Einstein's relativity.