There's more than one clearing firm in the market. The more money the clearing firm has, the more likely it'll be able to continue trading highly volatile stocks at high volume. Fidelity, as a huge financial firm that's been around for a while, had the assets to afford to keep trading it (and/or their customers weren't trading enough to put strain on them), robinhood (which has its own, small, clearing firm) couldn't, simple as that.