Most of the more intelligent bronies stay away from it, therefore, you can't find any intelligent bronies, and therefore, you simply say that they're all dumb.
Think of it this way.
There's 20 kittens in a box that says: Johns Kittens.
10 of the kittens are sick.
10 of the kittens are healthy.
The healthy kittens leave, leaving 10 sick kittens under a sign that says "Johns Kittens"
Thus, everyone thinks that John only sells sick kittens, because there's only sick kittens in the box and the healthy kittens left.
Same for the topic. Just replace kittens with bronies.