It's like when Doom: Eternal and Animal Crossing: New Horizons came out at the same time. People find it funny when two disparate properties release at the same time in the same space. Especially in the movie theater where double features are not uncommon, there's a lot of people who want to see both and think the high contrast in tone and style of movies will make for an interesting experience.

Personally I anticipate Oppenheimer being a pretty bleak experience so going immediately to Barbie after will be a fun palate cleanser