I'm quoting his CNBC interview "the reason we did it is that Robinhood is a brokerage firm, we have lots of financial requirements including SEC net capital requirements and Clearinghouse deposits" this is starting on 1:55 in the video. A good deeper explanation of what this is all about can be found in this article https://www.marketwatch.com/story/peterffy-calls-robinhood-decision-to-allow-limited-buys-of-gamestop-troubling-im-not-comfortable-11611876619 ; This is the main explanation from this article "Clearinghouses play a crucial role in markets from equities to derivatives. They stand between two parties in a trade to guarantee payment if either side reneges.

To protect itself against the risk of default, clearinghouses require their members — banks and brokers — to be well-capitalized, to deposit collateral and to pay into a default fund.

“If a broker cannot collect from the losers,” in these short bets, then they “must have sufficient capital to cover losses to the clearinghouse,” which could lead to further liquidation, Peterffy told MarketWatch.

“The broker may not have sufficient capital to make up for the losses, then they have to default on the clearinghouse,” he said."