I’m not sure there is just one reason as it varies–I think it depends on the person and the stage of change they are in… Some people are thinking about starting a program, but not really in the action stage yet. Others are in a program, but need motivation, support, and the desire to stay there. Some of the more common roadblocks could be time, money, support, desire, environmental, etc….
Many people may say time, but if you really want to do something you find the time. For others, it could be that they don’t like to exercise and need some fun, motivating ways to get movement into their day. Knowledge about what to do is another roadblock.
So many factors depending on the individual, but those are the ones I hear the most.