Valve did not say exactly why it got rid of its forced arbitration clause. But the company is currently being sued in a class action lawsuit in Washington state over the dominance of the Steam platform and over claims that it has overcharged for some games. The plaintiffs in that case actually went to arbitration and convinced an arbitrator that the forced arbitration clause should not apply to them, and were allowed to sue.

The plaintiffs in that case “retained separate counsel and mounted a sustained and ultimately successful challenge to the enforceability of Valve’s arbitration provision. Specifically, the named Plaintiffs won binding decisions from arbitrators rendering Valve’s arbitration provision unenforceable for both lack of notice and because it impermissibly seeks to bar public injunctive relief,” the class action lawsuit against Valve reads. Valve has not yet filed any arguments in that case.

Source: https://www.404media.co/steam-removes-forced-arbitration-clause-gamers-can-now-sue-valve/

The most likely reason is, in short, forced arbitration was a way to prevent consumers from collectively taking legal action, and attorneys ended up doing that anyway in a way that's even more costly to companies like Valve than class action lawsuits.

It's also caused some ugly press recently, and the US has already passed legislation prohibiting it in certain agreements. I'm not expecting it to be in terms of service agreements much longer.