I'm sure he was cut in large part due to the relatively low tolerance of these cases within the Steeler's organization. However, I suspect the fact that he was dead weight on the roster made the decision a lot easier.
I can't figure out why the Steeler's would use a draft pick to select a running back from a college like Florida, which is predominantly a pass oriented team that rarely uses blocking backs. Only 3 decent running backs from Florida ever amounted to much in the NFL and Rainey isn't one of them.
He was drafted to be a 3rd down guy and a kickoff returner. And he was decent at those jobs. He wasn't "dead weight". He actually was probably as valuable as Spinderella or Wallace. But he made a huge mistake, and will pay for it. Who knows, sometime in the future, he could become the next CJ2K. But it won't be with the Steelers.