Pure anecdote, but off the top of my head I could name at least ten women's Olympics gymnasts from the past twenty years and not a single swimmer other than Phellps and Lochte. The Bela Karolyi era was very, very good for US gymnasts and TV coverage followed. It seems to be the summer games analogue to figure skating in the winter games, Americans are consistently competitive and attention/coverage follows, particularly given the perceived demographics of Olympics viewers vis a vis other sporting events.