I see a few reasons:

• Universal appeal
• Sequel to a generally beloved animated movie that has gotten similar rave reviews.
• Has a built in audience of kids combined with teenagers/young adults that were kids when the first movie was released.
• A pretty slow year for movies before Inside Out 2 was released.
• No real competition until Despicable Me 4 was released like a month later. And IMO Inside Out 2 is just a straight up significantly better movie.

I have also heard stories about schools and psychology institutes getting tickets for teachers/members because the films are seen as a good entry point for understanding children’s emotions. But that was from randos online so I’m not sure how true that is.