They’re both big, popular, hyped up movies, that are completely opposite to one another, launching on the same day. That’s pretty much the jist of it.
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There's a bit more to it. The barbie movie was released on the same day as a fuck you to Nolan. Nolan had done most of his previous movies with warner bros but they had a falling out over Tenet. Nolan has strong feelings about cinemas.
It's the silliness of the contrast between them. Similar to the animal crossing and doom communities coming together and creating memes when their games launched on the same day.
They just happen to be the two biggest movie releases of the year this far. I think they beat all the Marvel movies and the new Indiana Jones in box office, which signals that serialized movies are not the huge phenomenon they used to be just a few years ago. They are also received very favorable reviews. So they are just getting a lot of positive attention.