Nine wins are great until you examine who they were against. Over the last few years, Nebraska successfully beat up on some bad teams and got walloped by anybody worth their salt.
Blowing out Purdue is fun, getting manhandled by Wisconsin, UCLA, Ohio State, etc. isn't. The way we lost some of those games, by not belonging on the field with the top teams, coupled with Bo's relationship with the AD and his overall attitude are why he was fired. Nobody cares about 9 wins if they're coming against Purdue, Northwestern, Illinois and Iowa. Getting blown out on national TV, over and over, overshadowed those victories.