Because it's popular. These people like to think of themselves as better--more enlightened--than the "masses" and if something is popular, that tells them it's no good. That way they don't have to overextend their meager intellects to actually judge for themselves the worthiness of anything. It doesn't take intelligence to be a cynic.
You are correct that using such a simple rule-of-thumb such as "if it's popular, it's no good" can be problematic, but I think the spirit of your comment goes a bit far in the other direction--the direction of majoritarianism.
Truth isn't determined by majorities, and one has to admit that the masses are pretty vulgar, even by the traditional standards of what constitutes mass vulgarity historically. I need only point to the baseness, and popularity, of cultural products like MTV's programming.
Conservatism has always been skeptical of the masses, and that can be found in texts like Ortega y Gasset's Revolt of the Masses. Of course, we live in an age where even conservatism has been watered-down and corrupted by mass culture. Some might say evidence of that can be found at Free Republic. ;)