Electra is the main character of this Greek tragedy. As we discuss in our "In a Nutshell," Sophocles's play treats the same myth that his older contemporary, Aeschylus, treated in his play The Libation Bearers, and that his contemporary Euripides treated in his own play Electra. The title makes it clear that, unlike Aeschylus, who focused on Orestes, Sophocles's play takes his sister Electra as the protagonist.
As we discuss in "Character Roles," there are many reasons that Electra, and not Orestes, is considered the protagonist of this play. First, there's the sheer number of lines and time on-stage. If you look quantitatively at the play, it's indisputable that Electra is the main character here. Next is the Chorus, who primarily converses with Electra and who sympathizes with Electra's plight (see "Tone" for more). Electra's roller-coaster ride of emotions makes up the real guts of the play – not Orestes's avenging action, which is limited to a quick scene at the end.
Because we experience the play from Electra's point of view, and not her brother's, we are focused more on the emotion and moral consequences of avenging rather than the action of avenging itself. What has the murder of Agamemnon done to Electra? How has she chosen to deal with it, especially in contrast to her sister, Chrysothemis? What arguments can Electra use to justify killing her mother and Aegisthus? These are the concerns of the play, not where or how Orestes murders the royal couple.