If you are strapped for time (as I am on the way to the Arizona Democratic Party state committee meeting), the answer to the last question in the headline is "moral imperative". They really, really do believe that nonsense. Even if tax breaks for the uber rich do nothing for the economy other than increasing income inequality, they still want them because taxes are immoral. So all the evidence to the contrary will not shake their faith in cutting taxes for the wealthy. All the additional nonsense about 4% growth is a cover story for that conviction.
Hence Bush's plan (summarized here in Paul Waldman's Plum Line piece at the Washington Post).