When the gap between the wealthy and the poor grows too wide, it is impossible for the economy to grow. A very small percentage of the population is plugging a lot of money that can't be reciprocated back into the system by the poor because they simply cannot afford the goods or services that the wealthy are selling. The end result is a stagnate economy.
We all talk about equity wealth distribution but in all honesty it would be counter productive to sustainable development. Many of the wealthy are comprised of individuals that had the innovation and motivation to develop platforms to drive commerce and industry. They did this in part due to an inner need and drive but the accumulation of wealth is a very powerful motivating factor. Take away the ability to accumulate wealth and there will be no reason to push forward.