King Lear favors Goneril and Regan because their professions of love are so much more extravagant than Cordelia's. It is obvious to the audience that the two older sisters are just lying because they are motivated by greed and not by love. Lear has foolishly promised to give the best portion of his kingdom to the daughter who loves him the most. Goneril and Regan are competing for land, power, prestige, money--their insincerity and greediness are obviouus, but they don't really care what anyone thinks as long as they can fool their old father. Cordelia really loves him and does not want to lie to him; it is not in her character to do so. She reveals her misgivings in several asides while her wicked sisters are professing the love they do not feel. Cordelia ends up getting nothing, and her two sisters divide her third of their father's realm.
You could argue that the irony of Lear's acceptance of what his first two daughters say and his inability to understand of how they say it is a symptom of undiagnosed autism.