It's tough to be a regional infrastructure-based provider of mobile service these days, given the speed with which the technology is moving, and the scale needed to make decent financial returns. In wireless, as in video, volume makes a difference.
Cable companies have had quite a long history of dabbling in spectrum and working with Sprint for wireless services, and nothing too significant ever has developed from those efforts. For whatever reason, cable operators have been most successful offering new services over their existing broadband networks.
One might speculate on the cultural reasons for that state of affairs, but among the possible explanations is that cable executives never had felt comfortable relying on other networks to deliver their services. Whether that has created some managerial impediment is a question.