One explanation I have read is that when tax rates on the high ends of income are lowered, it tends to encourage speculation, leading to bubbles in the economy. And we know that bubble will eventually burst and there is hell to pay.
Now the correlation between tax rates and unemployment is something you can look up yourself, but I will site a few examples, just to make the point.
In 1980, the top rate for individual taxes was 70% and unemployment was at 6% and it had been falling since 1975. In 1981 the top tax rate for individuals was cut to 50%. This began a series of tax cuts for individuals, corporations, and capital gains. The unemployment rate rose, spiking at over 10% in 1983.