I heard this yesterday, and it really made sense (and sorry if it was mentioned before)--
Disney+ is the reason why Lightyear hasn't done well. Everyone knows it will be on Disney+ in about a month, so why spend the money to see it in a theater since so many people who are the target audience have Disney+?
Minions on the other hand is in theaters, and no one feels like they know it will be out on a streaming service or which one. It could show up in a month or it could be Christmas before it's out on a streamer.
Disney+'s success is killing the box office of their new releases.