I'm a Ruben Sim fan, he makes good videos, you're probably too young to watch them though.
I think a TL;DR would be something like "ROBLOX's moderation is inept enough to the point where it takes 10 minutes of light effort to find children to sext", in Minecraft I don't even know where you'd start to do smthn like that, I mean maybe there is a Minecraft pedophile ring and its just yet to be discovered but its a lot of work to get the stuff you said in Minecraft compared to doing the same thing on ROBLOX, to me its just the ease of access, and once again the free accounts. If someone were to get completely groomed on MC they would probably take it to discord while its a one stop shop on ROBLOX. At this point its just the most popular choice so the mobs of pedos all choose ROBLOX, and they make it easier and easier with stuff like public UGC.