The RWNM, and Republicans in general, have a magic formula for any problem.
Back in the days it was passed, the Bush tax cut for the rich was going to encourage a huge increase in savings. Not only were millionairs going to save enough for them to buy the extra bonds that the US Treasury had to sell to raise the money that wasn't going to be collected in taxes, they were going to save much more, too. (Remember that "saving" means not consuming.) They were going to put that extra savings into expanding the manufacturing capacity. We were all going to be better off. (And it would be easy to repay those bonds from the extra taxes from all that new production.)
Now, though, we have a lot more capacity than we need. The need isn't for new capacity, it's for consumption. And now, magically, the rich are responding to that tas cut by consuming much more. We can't afford to cut their taxes, because that would decrease some of the consumption that we need right now.