By limiting the action and the entire story to a single act, Susan Glaspell avoids introducing unnecessary obstacles to the central story. The play concerns the isolation and marginalization of women, and the single act symbolizes the small amount of attention that society pays to women's issues. With more acts, the story and the denouemont would require extension, and the themes of the play would be lost in the need to create tension with other characters and events. The single-act is best able to focus entirely on the underlying themes and important events that are revealed by dialogue:

MRS. HALE: [Examining the skirt.] Wright was close. I think maybe that's why she kept so much to herself. She didn't even belong to the Ladies Aid. I suppose she felt she couldn't do her part, and then you don't enjoy things when you feel shabby. She used to wear pretty clothes and be lively, when she was Minnie Foster, one of the town girls singing in the choir. But that -- oh, that was thirty years ago.
(Glaspell, Trifles, etext.virginia.edu)

This piece of dialogue shows the effect that marriage had on Minnie, and how she isolated herself afterwards; the single act is "isolated" from prologue and epilogue, without the need to overwrite the story. Additionally, the single act is sparse, and devoid of music, poetry, and spectacle; Minnie's life was equally sparse, without any method of expressing herself emotionally or creatively.