Disclaimer: I am not endorsing 9% rates, or 90% rates. I'm simply turning up the contrast button to eleven to illustrate the concept.
Here's how it works:
If you or I buy a $1000 personal computer for fun, it costs us $1000 out of pocket.
If I'm in the 40% marginal bracket and I need a $1000 business computer, it'll cost me a net of $600 after taxes.
If the tax rate is 90%, it'll cost me $100, net.
If the tax rate is 9%, it'll cost me $910, net.
In which scenario am I more likely to reinvest in my business? Which rate incentivizes me to take money out and pay the small hit?
If rates go from 35% to 20%, I'm less likely to reinvest, since---comparatively---my incentive of a tax break has decreased.