Because there are better brokerages out there.
> in the face of the partners they rely
You mean their actual customers. There are brokerages where the incentives are not perversed like this.
> did the best they could by still allowing selling
Assuming they acted in good faith to do their "best", their CEO was still incompetent failing to anticipate to liquidity issues
> what happened on one specific day to one specific stock
They halted buying on more than GME.