I think the difference is that dogs and poor children aren’t known for trying to convert people to be like them or for disapproving of other people’s identities and lifestyle choices, but proselytising is a fundamental part of Christianity for many people and many of us have experienced bible-based judgement for some aspect of our lives.
I wouldn’t personally think everyone who sent me a Bible verse was doing so to teach me about Christianity, but I can understand why others would. For what it’s worth, I have received less than 200 official cards but still at least 4 of those have explicitly tried to teach me about the virtues of letting Jesus into my heart. (I sometimes wonder if this is because I specifically mention other religions on my profile, or is this the average Postcrossing experience?)