I would say that there is no one most influential factor. The key to success is in balancing many factors. That is true for all architecture, and especially public architecture.
Reaching people who really need public transit and taking them where they need to go is key. This means serving the poor, and setting up drive/park/ride systems for middle-class people.
Raising enough money and getting social and bureaucratic commitment will take years, if not decades. So one must plan well in advance. Some of the public transit systems developing in growing South American cities are excellent models right now.