why cosmic background radiation is not ether? I mean it's everywhere and it' a radiation then we can measure Doppler effect by moving with a velocity.
The luminiferous aether was, by definition, a hypothesized medium that was needed for electromagnetic waves to propagate through space. The cosmic microwave background isn't needed for photons to move; indeed, they move through space even if one removes (shields) the cosmic microwave radiation. So that's why CMS isn't luminiferous aether.
On the other hand, the aether also made a particular prediction, the existence of a preferred reference frame. In this sense, the CMB plays the same role as the aether. Cosmologists use the reference frame associated with the CMB as the preferred coordinate system. However, this ability to pick a "preferred" coordinate system depends on the environment – the CMB is just some property of the environment that could possibly be different as well (e.g. if you shield it). In this respect, it still differs from the luminiferous aether that couldn't have been shielded and that guaranteed the existence of a preferred coordinate system in any situation, regardless of details of the environment.
There are a couple of misconceptions here. The CMB radiation is thought to be the (historical) light left over after the big bang. Due to space expansion, it has been red-shifted towards the microwave region of spectrum. While on the other hand, ether is the medium required for light to travel when it was thought as a longitudinal pressure wave (like sound)...
In light of your question, you're saying that the red-shifted EM radiation acts as a medium for light to travel... - which is certainly untrue..!
Why cosmic background is no ether is not a question it is a statement.
A question would be is the cosmic background an ether?
For one thing it is an inertial system. This inertial system can be found by anyone in the universe. One can find(calculate) his or her velocity to it.
Since a inertial system is sufficiant to describe physical phenomena in it has all the properties to state that Lightvelocity is constant c in it.
If Michelson knew what we now know about Cosmic Background Radiation he would not have build his large interferrometers to measure the velocity of the solar system to the universe and special relativity would not have emerged.