Last week’s blue wave means that Donald Trump will go into the 2020 election with only one major legislative achievement: a big tax cut for corporations and the wealthy. Still, that tax cut was supposed to accomplish big things. Republicans thought it would give them a big electoral boost, and they predicted dramatic economic gains. What they got instead, however, was a big fizzle.

The political payoff, of course, never arrived. And the economic results have been disappointing. True, we’ve had two quarters of fairly fast economic growth, but such growth spurts are fairly common — there was a substantially bigger spurt in 2014, and hardly anyone noticed. And this growth was driven largely by consumer spending and, surprise, government spending, which wasn’t what the tax cutters promised.

Meanwhile, there’s no sign of the vast investment boom the law’s backers promised. Corporations have used the tax cut’s proceeds largely to buy back their own stock rather than to add jobs and expand capacity.

But why have the tax cut’s impacts been so minimal? Leave aside the glitch-filled changes in individual taxes, which will keep accountants busy for years; the core of the bill was a huge cut in corporate taxes. Why hasn’t this done more to increase investment?