The reason the buy button was turned off is because when you buy a stock on any brokerage, you don't immediately own it, the shares still have to go through the clearinghouse. The brokerage needs to post collateral at the clearinghouse while they wait for the shares to deliver. The demand for GME was unprecedented, the clearinghouse raised their collateral requirements, and Robinhood didn't have the money to post collateral, even after raising a billion dollars overnight. It's very simple. There was an entire Congressional inquiry into this, which you can read into instead of spreading baseless conspiracy theories on HN.