As for why Tate wasn't brought in sooner. Does anyone remember that the Steelers did "due diligence" on him when he was released by the Browns and Blount was released here?
http://fantasy.usatoday.com/news/interested-in-ben-tate
He chose instead to meet with the Vikings and ultimately received a contract. He stayed with the Vikings until December 23 (one week ago).
http://network.yardbarker.com/all_sports/article_external/minnesota_vikings_release_ben_tate/17965850?
linksrc=story_player_ben_tate__nfl_auto_module_head_17965850
Bell got hurt 2 days ago and Tate is being brought in. It's not like they could have signed him from the Vikings when he was under contract with them.