To be upstaged now means to lose due credit to a lesser person.
In the theater, “upstage” refers to the back of the stage, which at one time was built higher than the front.
This was because the theater floor was flat, and a slanted stage gave a better view of all the actors.
Plays were crafted placing noble characters at the rear, where they appeared higher and more regal, even though they might have fewer lines than the others.
We say we’ve been “upstaged” when someone else grabs all the attention.