It's because people who are obsessed with moralfagging about those topics tend to do it constantly with zero pushback because nobody disagrees, then turn it around when people tell them it's time to shut up because they made their point 20 minutes ago and come back with some variation on at the very least implying you're less virtuous than they are because you don't want to hear an hour and a half more sperging about child porn, which is a one way ticket to get anybody to hate you.
They're as bad as loli-spergs in their intensity but worse in my mind because they've picked a position on a topic nobody normal disagrees about and use that as a cudgel when people "don't care enough" for their tastes. At least you can tell a loli-sperg they jerk it to cartoon children and they really don't have a good response which is your cue to ghost them.
Edit: And yes, that also applies to trying to leverage people to personal army for your causes using moralfagging about that topic as the lever. Very few things cause more resentment than using that topic, which nobody normal disagrees on, to try and force someone to do something they don't want to do because saying "no" can be spin-cycled into "pedo apologia". Both parties know the score when this happens and the party being leveraged understands immediately that is a huge dick move.