Generally scientists enter their field with the goal of doing important new research and accept as their colleagues those with similar motivation. Ttherefore, when any scientist wins renown as an expounder of science to general audiences, most other scientists conclude that this popularizer should no longer be regarded as a true colleague.
The explanation offered above for the low esteem in which scientific popularizers are held by research scientists assumes that research scientists believe that those who are well known as popularizers of science are not motivated to do important new research.