This collateral requirement goes up when a stock becomes heavily traded, or more volatile.
Because of the insane volume & volatility of GME, RH's cushion of collateral was exhausted (And they've now replenished it, by borrowing & raising money.)
The reason that sells were permitted to execute was because each sell by one of their users reduces the collateral requirements in their settlement accounts. (So the settlement houses allowed sell orders to go through.)
The tl;dr is - when you are trading through a discount broker, you get a discount service. Serious brokerages, that are used by institutional investors don't have these problems, but they also charge a lot more for their services.