Why have trickle-down economics failed to trickle down to the middle-class and poor in recent years? Trickle-down economics, also referred to as trickle-down theory, is an economic theory that advocates reducing taxes on businesses and the wealthy in society as a means to stimulate business investment in the short term and benefit society at large in the long term. 

When it works, wealth created by the rich will trickle down to the middle-class and poor. Hence, the name trickle-down economics.

https://en.wikipedia.org/wiki/Trickle-down_economics

Strangely, one of the richest man in U.S, Warren Buffett, does not seem to think so. Why have trickle-down economics failed to trickle down to the middle-class and poor in recent years?

https://www.cnbc.com/2018/01/04/warren-buffett-on-the-failure-of-trickle-down-economics.html


  Buffett says there is a problem with that economic system, which made
  him a king: Many individuals suffer even as those at the top prosper
  wildly.
  
  He points to the Forbes 400, which lists the wealthiest Americans.
  "Between the first computation in 1982 and today, the wealth of the
  400 increased 29-fold — from $93 billion to $2.7 trillion — while many
  millions of hardworking citizens remained stuck on an economic
  treadmill. During this period, the tsunami of wealth didn't trickle
  down. It surged upward."
Trickle-down economics are widely criticized and many believe that it doesn't work. Asher Edelmanon explains brilliantly in the first 2min of this video (or in the written version here).

The basic logic is that, in practice, doing this only leads to a higher accumulation of wealth by the wealthy.