Anakin is the only one de-aged because Anakin hasn't lived the past 30 or so years - Anakin was more or less killed by Darth Vader. He left behind the parts that made him Anakin Skywalker when he lead the clones into the Temple.

When he was redeemed by Luke, he reclaimed those parts of him. His sense of honor, pride, decency, love, etc were all restored. His mental self-image would likely have returned to the way it had been when he last felt that way - as a young man.

The out-of-universe reason? Controversy sells better. Also, Shaw doesn't look a thing like Anakin ever did (on screen), so there would be a major disconnect for first-time viewers if he weren't replaced by Hayden.