That's because individual investors tend to encounter short-sellers as anonymous online posters who say nasty things about stocks they own.
Nobody likes a spoilsport.
The bulls then accuse the shorts of telling lies or of being un-American in betting against a company's stock. Some short-sellers do come off like arrogant jerks, so it can be easy to dislike them. But even polite, calmly rational shorts get flamed mercilessly. Bulls who get mad at short-sellers, though, are almost always too emotionally wrapped up in their investment to recognize the merits of the bear argument. They often know a lot more than professional sell-side analysts know or care to admit.