Because people want to do what they want to do without any pesky reminders that certain things are wrong. So instead of politely acknowledging a tactful reminder that what they are doing might not be a grand idea, they get all huffy and accuse the reminding party of being preachy and judgmental.
But to be fair, there is also a proper and tactful way of pointing out to someone that they should rethink what they’re doing. Going up to them and loudly saying “that’s haraam and/or you’re going to hell” in a crowded room isn’t the way to do it.