Aeschylus's Agamemnon is named after its tragic hero, King Agamemnon of Argos. What's a tragic hero? Tragedies typically tell the story of a great man (sometimes woman) who gets cut down to size; this initially successful but ill-fated person is usually referred to as the "tragic hero." It's actually pretty common for the names of tragic heroes to appear in the titles of the plays centered on them; thus, Oedipus is the tragic hero of Sophocles's Oedipus the King, or, in more modern times, Hamlet is the tragic hero of Shakespeare's Hamlet, and King Lear is the tragic hero of his King Lear. So, Aeschylus's Agamemnon seems to fit pretty nicely into this pattern.
That said, isn't it interesting that Aeschylus's tragic hero is only onstage for one scene of the play, and basically all he does is show up and get killed? In terms of people who actually do things and influence events, you might say that Clytemnestra is the more important figure. Should Aeschylus have named the play after her? As it stands, it seems as if the title of the play creates a certain set of expectations in the audience, but these expectations don't end up being met. Why do you think Aeschylus might have done it this way?