"Because it offers a rationale for lower taxes on the wealthy":
Zombies of Voodoo Economics, by Paul Krugman, NY Times: According to many reports, Donald Trump is getting frantic as his administration nears the 100-day mark. It’s an arbitrary line in the sand, but one he himself touted in many pre-inauguration boasts. And it will be an occasion for numerous articles detailing how little of substance he has actually accomplished. ...
Mr. Trump sold himself to voters as unorthodox as well as effective. He was going to be a different kind of president, a consummate deal-maker who would transcend the usual ideological divide. His supporters should therefore be dismayed, not just by his failure to actually close any deals, but by the fact that he evidently has no new ideas to offer, just the same old snake oil the right has been peddling for decades.
We saw that on Trumpcare... And now we’re seeing it on taxes. ... Whatever the details, Trumptax will be a big exercise in fantasy economics.
How do we know this? Last week Stephen Mnuchin, the Treasury secretary, told a financial industry audience that “the plan will pay for itself with growth.” And we all know what that means..., history offers not a shred of support for faith in the pro-growth effects of tax cuts..., supply-side economics is a classic example of a zombie doctrine: a view that should have been killed by the evidence long ago... Why, then, does it persist? Because it offers a rationale for lower taxes on the wealthy...
Still, Donald Trump was supposed to be different. Guess what: he isn’t.
To be fair, it’s not clear whether Mr. Trump really believes in right-wing economic orthodoxy. He may just be looking for something, anything, he can call a win — and it’s a lot easier to come up with a tax reform plan if you don’t try to make things add up, if you just assume that extra growth and the revenue it brings will materialize out of thin air.
We might also note that a man who insists that he won the popular vote he lost, who insists that crime is at a record high when it’s at a record low, doesn’t need a fancy doctrine to claim that his budget adds up when it doesn’t.
Still, the fact is that the Trump agenda so far is absolutely indistinguishable from what one might have expected from, say, Ted Cruz. It’s just voodoo with extra bad math. Was that what his supporters expected?