The short play Trifles, also known as the short story A Jury of Her Peers, does in fact have as its main character Minnie Wright. The action of the story takes place after the strangulation death of her husband. Minnie has been arrested for the murder, and the sheriff, his wife, and two witnesses are inspecting the scene of the crime.
Although Minnie never appears in the play/story, she is the main character because all of the action revolves around her. We learn of her youth, when she was a carefree Minnie Foster. Later, after she marries a stern, miserly man, all joy is sapped from her life. We discover that she had a bird, which we imagine she cherished, enjoying its cheery song and exchanging affection with it as she could not her with her cold husband.
The two women in the kitchen discover that the bird has had its neck broken. We are meant to believe that Mr. Wright killed it, an act of cruelty that apparently drove Minnie to murder the man.
By learning these aspects of the story from "The Jury of Her Peers"--the two women, the reader develops great sympathy for the isolated Minnie. Her story is truly tragic. The author's clever technique of revealing the "trifles" of Minnie's life through the eyes of her fellow farm wives in the small town, we understand her much better than would have been possible from her own narration.
Your question is why does Glaspell keep Minnie Wright off stage if that is her main character?
Minnie Wright is not on stage because she is in jail. The setting is in her house after the arrest has taken place and John Wright's body remains upstairs.
Although she is the main character, she is not physically on stage.
I don't know why Glaspell keeps Minnie Wright off stage other than it adds a bit of depth to the story - meaning she is not there to defend herself, but the audience is there to piece it together.