"if"
One of the pillars of trickle-down economics is a tax cut for the rich. The idea is that rich people will buy more capital (expand the economy) if they have excess money. Then they'll hire people to work that capital, spreading the money around to the poor.
The reason this doesn't work is because it's not rational to build a factory if you don't expect people to buy the products you make. It doesn't matter how much discretionary income they have. It's better to stay liquid and wait for a good opportunity than throw good money into a bad market.
Incidentally, the way you avoid this problem is through business taxes. Unlike you and me, businesses are taxed on their profits (revenues minus expenses - and before you say it, no, until you can deduct rent and food, your petty deductions aren't the same thing). A high corporate tax rate, even a punitively high one, means corporations "use it or lose it": they have a strong incentive to spend their money today, and expand the economy today, in anticipation of future bull markets - and, by that very expenditure, end the crisis.
Without that stick, rich people sit on stagnant cash forever, waiting for an upturn that will never come.