In television, cathode rays hit screen coated with certain fluorescent or phosphorescent material. Even though we are using same T.V for years it gives the picture with clarity. Why does the power of glowing of the material doesn't decrease even we are using for years?

The phosphorent material doesn’t loose the power of glowing even after its prolonged use because of the fact that it is actual cooling off(making a transition from excited state to ground state) after being hit by the cathode rays from picture tube.
So cathode rays excite the phosphorent material and when they cool off they give away light.