Minnie Wright is kept offstage because she functions more as a thematic centerpiece of the play than a dynamic character. We do not need to know anything about her individual personality and quirks. In fact, these forms of humanization would detract from the overall theme of oppression and the feminist lens through which we the readers view it. We need only know her history.
The action of the play is centered around the murder of Mr. Wright and the suspicion of the town's court attorney, sheriff, and a neighbor of the Wrights. As they search with cold calculation, the wives of the sheriff and the neighbor discover what the men could not, Minnie's beloved pet bird. It has had its neck broken, presumably by the late Mr. Wright, causing Minnie to snap. Understanding male oppression all too well, the wives hide the evidence.
Minnie Wright is kept offstage because the audience is intended to wonder at her actions. The audience is not sure if or why Mrs. Wright killed her husband. Glaspell puts the audience in the same position as Mrs. Hale and Mrs. Peters, as well as the male authorities, all of whom have to piece together what happened in the Wrights' household.
The audience witnesses Mrs. Hale and Mrs. Peters putting together the clues they find, such as Mrs. Wright's slipped stitches on her quilt and her dead bird, whose throat has been broken. Like the audience, they put together these clues and figure out that Mrs. Wright suffered from loneliness and an abusive marriage. Glaspell wants the audience to put together the reality of Mrs. Wright's life along with the female characters in the play.
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.