Again, because Roth didn't exist in GF. In Roth's soliloquy in II, Puzo and Coppola were "reverse-engineering" Roth and his relationship with Moe.

BUT, if we project Roth as an unseen presence in GF, we could imagine that Roth didn't try to intervene with Michael because Moe--"headstrong, talking loud, saying styupid things"--had dug his own grave, and intervention by Roth would have been useless--fatal. Also, Roth might even then have been negotiating with Michael for bigger things, so Moe's killing "had nothing to do with business"--the business Roth was negotiating or envisioning with Michael. Also, by muscling Moe, Michael was signaling his intent to be a big player in Nevada--Roth's turf. Roth may have known, even then, that he'd have to kill Michael to stop him from taking over. By "letting it go" (Moe's murder), Roth, ironically, was adopting the Corleone method of appearing weak, the better to lull his enemy and strike when his foe's guard was down.