The name comes from the fact that we, as fans of fictional stories, like to treat the worlds our favorite stories take place in as though they have their own, objective existence independent of us.
Yes, technically speaking, these fictional worlds only exist as data on recording devices and impressions in people's minds. But it is still very, very common for fans of a story to talk about it as though it were real. It doesn't take much digging to find plenty of Lost fans arguing about what the Island really was. You could say, "The Island isn't really anything; aside from the fact that it's fictitious, the show never said what it really was, so the answer doesn't exist." But where's the fun in that? It makes for much more interesting conversations to act as though there's some parallel universe where the events depicted in Lost actually happened and to wonder about those parts of the story's world that we didn't get to see.
Since fans often treat stories this way, how they view Retcons is affected. Think of it like this: A historian discovers absolute proof that Julius Caesar was actually Chinese. This would contradict a great many historical assumptions about old king Caesar. Does this mean the historian has altered the cosmos, warped reality in order to change Caesar's ethnicity? No, it just means our historical records were wrong. The universe is the same as it ever was; we've just become better informed about it.
That's how a lot of fans approach Retcons. Suppose a character dies in the first episode of a TV series, but in episode four they're suddenly up and walking again, explaining their death away by saying, "I Got Better." The way Fandom tends to view this sort of thing, when we we saw this character die in episode one, they were actually just Faking the Dead, or they came Back from the Dead offscreen. The world of the story hasn't changed; it's just that what we thought we knew about it was wrong.
A Cosmic Retcon, however, because it happens in-universe, must therefore change the universe (a.k.a. cosmos). It's not simply our knowledge of the fictional world being changed; we actually get to see the world's history being rewritten. Where a regular Ret Con goes, "Yes, you thought this character died, but they actually didn't, " a Cosmic Retcon goes, "Yes, this character did die, but that's not true anymore." This isn't a historian discovering Julius Ceaser was Chinese; this is someone going back in time and switching the newborn Ceaser with a Chinese baby. The former simply alters our knowledge of history; the latter alters history itself.