First off, there's the standard Internet thing, where anonymity+audience makes people act like jerks because they have freedom to say whatever they want with (typically) no consequences, so they indulge in that. And indeed, you'll find people saying mean things in most places, even in communities that are all vanilla heterosexual white Christian men, there will be people who find reasons to bash each other if such behavior is tolerated. Examples already mentioned are things like sports rivalries, different gym memberships, school rivalries, etc.
But we might ask why does diaper play, specifically, upset a good number of people? I think there are a few reasons. First, it's transgressing a social taboo. Diapers are for babies and pretty much everyone in developed society goes through a stage early in life where they're told not to wear diapers anymore and encouraged to use the potty instead (and often teased for it if they're slow at the process). Intentionally transgressing social norms of any sort tends to make people, both the transgressor and the people around them, really, deeply uncomfortable. Here, check out
this Wikipedia article about some psychologists who studied this stuff, it's cool.
Second, diapers, specifically, have a certain gross out element for people. Pee and poop make a lot of people uncomfortable. Even without the ABDL community, people have totally different tolerance levels there. So when somebody who isn't ABDL is reading about someone using diapers, it grosses them out and they're quick to assume that since they're grossed out, other people should be too and anyone who isn't is wrong and different.
Third, maturity or immaturity are extremely loaded words in society. People use them to refer to a person's overall social health and ability to be responsible, live well, get things done, and show up on time to things. So acting "immature" in the ABDL way tends to create associations for a lot of people with "immature" meaning a person who isn't fulfilling their social responsibilities. That's why a lot of the comments take the form of "oh, they're sitting there wetting themselves instead of being productive" or similar sentiment with more crude language.
And lastly, ABDL is rare and the community is small, so it's very rare that somebody randomly stumbling on an ABDL video is going to personally know anybody who is ABDL. Especially since most of us are also very secretive, for obvious reasons. Knowing someone individually is the best way to stop a person making fun of other people because you'll think of that individual when making a blanket statement and check yourself. But that doesn't happen much in the ABDL context.