How often do you see a joke about Orthodox Jew side curls, or it used as shorthand to identify someone as Jewish?
Or a yarmulke being used in a joking matter or for stereotyping a TV character, when it's intended as a sign of faith. There are entire movies about Rumspringa as this wild, debaucherous time of crazy sin, when the reality is clearly much different. The Muslim call to prayer and method of praying is often subtly mocked in media or seen as an annoyance (especially when broadcast over loudspeaker) instead of trying to help people keep the covenants they believe are integral to their salvation.
It's because people don't understand the difference between something being sacred and just being unique to one culture/religion or weird in that regard. And frankly, we don't always do a good job of that.
At work, one of my coworkers asked the other LDS guy if it was true that we actually have 'special underwear' and the other guy stuttered, got flustered and responded "I can't talk to you about that. It's not something we discuss". This of course just piques curiosity and makes it seem more secretive than sacred.
I then took my coworker and explained that when we go to the temple, we make covenants to live our lives in certain appropriate ways, and that garments (as one purpose) serve to be with us always and to remind us of those covenants. At which point my coworker went from thinking "Mormons are a bunch of weirdos" to "ok, maybe not how I'd handle it, but that kind of makes sense. Fair enough"