Having every other ship named USS Yorktown would get confusing.
Aircraft carriers should carry names that have appropriate historical weight to them. Carl Vinson/John Stennis/Gerald Ford/George Bush do NOT qualify as such. Naming a smaller ship after them would be much more appropriate. Naming any ship after Gabrielle Giffords is just silly. She's not a figure of any historical significance at all. What happened to her was tragic, but she's really no more than a flavor of the week celebrity.