When the preferential rate for capital gains was established there was a justification for it.
The total amount of money available in the economy was only a few thousand dollars per capita. Every available dollar which could be diverted into badly-needed capital investment would be worth the revenues lost. Tax incentives for capital investment were needed because capital investment was risky - the tax break was intended to offset the prospect of loss by the investor.
Today not one of these factors is the same.
Today there's hundreds or thousands of times the amount of capital that there used to be. The Federal Reserve in the last thirty years has created so much money that it's stopped keeping track of it [the M2] because they don't want the public to know how much it is.
There is so much money under management and in deposit all over the world that there is a colossal surplus of it. There are not enough capital investments of merit available for all the money that's lying around.
Furthermore the risks of capital investment no longer warrant a tax break.
Today's capitalists have no other way of making money other than to "invest" it. They will place it somewhere to be absorbed by some asset or business whether there is a preferential tax treatment or not. Their philosophy is like that of a casino gambler. They bet for or against certain markets and outcomes that have no tangible benefit to the economy. Real investment is only a small fraction of capital placed here or there.
Today's capital gains tax break is like giving preferential tax treatment to casino winnings.