Likely because they were the first, and the rest caved in shortly thereafter (though in some cases, it was the clearing houses who stopped taking orders). In RH's case, it was RH's management itself that made the call, which makes them more accountable to the decision. They can't point to their clearing house and say, "don't blame us: it was the clearing house".

I don't think they were the first. I think TDA for example restricted people from buying GME the day before.

Also, RH's initial communication about the matter was full of considerable shit -- moreso than other brokerages.