Long post, but here's all I can really think of this whole thing that's worth saying. This is just how tumblr happens.
Are they trying to emulate the relationship of Cool Cat and Daddy Derek?
Another thing I'm confussed is why ddlg is so strong on TUMBLR of all sakes where it's a bastion of radical feminism, no means no, anti patriarchy... etc How this shit is so strong over there?
I know I'm a bit late to the boat on this question, but Tumblr is honestly the perfect place for this kind of thing to flourish. Tumblr is a great hugbox builder because it's so easy for everyone to ignore each other. You don't have to see anything from outside of your community or be exposed to anything that contradicts your beliefs. If you are, you intake so much reinforcement of your beliefs that it's hard to let the occasional criticism that seeps in bother you.
Even people who dislike this shit become enablers through usage of blacklists and tumblr savior. If "cummies" or "ddlg" triggers you too much, you can hide it altogether and pretend this all doesn't exist. This takes away a chunk of the people who object the DDLG community, minimalizing their exposure to being called out. And those who choose to fight about it are either dedicated blogs that quickly get blocked or incidental opponents who get caught up for one reason or another (like an a piece of art being turned into a roleplay thread) and get fatigued trying to futilely argue. People can try to pick fights with littles and daddys, but again the community is going to be intaking so much of their friends' support that no stray voices will sway them. Plus, unless you specifically message the other party, then you're saying things in a post that is again reaching more of your own followers than the people you're trying to sway. I've even seen posts advocating cleanly cutting off anyone who's into DDLG and just ostracizing them. (Not that I think arguing with them would wind ubeing worth it)
Effectively, there's a wall between these polarized sides of tumblr and on each side everyone is just circlejerking. On one side you have "DAE cummies" memes and "people who like ddlg should slit their wrists", and on the other side you have people who are so absorbed by their questionable fetishes that they just can't turn back, and that's mixed with any potential mental illnesses and abusive power structures that might be in play. And the absurdity only helps, I imagine. Just like we don't take SJWs seriously when they talk about how white men are awful and deserve to die, I don't imagine that the DDLG community takes it seriously when they say "anyone who's into DDLG deserves to die".
And of course, the culture of tumblr actually lends itself to this well. "Kinkshaming" has become a joke, but not too long ago it was a legitimate controversy. People would fight over whether or not what you do in the bedroom is anybody else's business, and of course Tumblr can only reach a blanket conclusion on something. Either everything is okay or nothing is okay. And no matter how different two fetishes might be, they must be judged the same way. It's especially weird with DDLG, since some people probably see it as so close to BDSM. And there's a whole conundrum about being accepting of things like BDSM and trying to juggle the de-stigmatization of it with the reality that abuse can and does occur with it sometimes. But again, a blanket conclusion is easier. And, well, DDLG is gross so nobody wants to look into it too much. It's much easier to just say it's a dressed up cute BDSM than it is to read through blogs of widdle twee yee owds euphemistically asking to be coerced into sex acts by "Daddy". Your typical tumblr user can't stomach that shit, so as they do with most everything else they decide not to think about it for more than a few seconds. But in some circles you have to be polite about it, which is why when DDLG users see people complain they just say "oh, well, i'm sorry

".
Then you have everyone using it as a means to gain reputation. Either you're so moral because you condemn pedophilia, or you're so accepting and understanding of how a kink is just a kink. Everyone wants to one-up one another, and more importantly everyone wants to survive in the cut-throat Tumblr culture. You have to pick a side, and you can never change it or else you'll lose any chance of acceptance by either. So everyone takes a stand and says what they think will make themselves look best in the crowd they're already in. Asspats go around and that's the end of that. Everyone wants to get back to their aesthetic posting and get away from this TRIGGERING material as quickly as they can.
Nobody wants to touch CSA with a ten-foot-pole either. If an alleged CSA survivor says something is ok, are you allowed to judge? Is their judgment final? What happens when other CSA survivors have a different opinion? And all these other conundrums that cause tumblr users to overload into a shock and do the best they can to ignore it. And then a lot of these kids are throwing their queerness in there, just to make the waters murkier. Do you want to be the asshole who oppresses a #
queer little? Muh coping strategies!
Basically:
Tumblr's culture just lends itself to this kind of thing beneath the surface. It's a lot like how so many other conflicting cultures exist on tumblr, or even just undesirables. Remember all those bitches who shoplifted because FUCK CAPITALISM and largely went under the radar at first because they stayed to their cliques? Or even more simply, how there can be so many "doubles" of a character kin who all do so well on tumblr despite exploding whenever they come into contact? And it's exactly how people who are called out as known "abusers" (legitimately or illegitimately) often still manage to slink around. Tumblr is full of social barriers and people who don't peek over the neighbor's fence. I'm sure you could find a community for anything if you look deep enough.
Nobody wants to be responsible for dealing with the DDLG community except for a few single-purpose blogs. Tumblr has no proper reporting system, so anyone who's bothered enough by it has to go out and e-mail tumblr and probably be ignored. Does tumblr staff bother to monitor things like the DDLG tags? Probably not. (I wonder how much a person would have to be paid to got through that...)
But the site's mechanics are the big draw too. What other major sites give you that kind of isolation? Privacy with a community? You can't use Facebook, lest your real mommy and daddy find out about your deviant desire to be sexually manipulated by a father figure.
I'm sure it's not exclusive to tumblr, but there's enough reason for it to flourish there. There's gotta be all kinds of sites for this shit out there, and I'm not going to be the one to look for them. That weird disgusting photo comic was more than enough for a lifetime.