There are several reasons it's hard to maintain an exercise program.
Explanation:
It'll be easier if I just do a bullet-point list:
No access to a gymBuilding off access to a gym--they may have access to a gym but no transportation or means of getting there. No time--either gyms aren't open at the right time or work/life gets in the wayNo motivationNo one to exercise with (some people need a gym buddy to help keep motivation up)Money--most gyms require a membershipWhen most people start an exercise program, especially to lose weight, they can see dramatic drops in their weight, but eventually this can plateau, which then turns them off exerciseAlso people can see a lot of gains in strength, but unless you structure your workout properly and increase the load/intensity at the right time, that too will taper off and once you can't see the crazy gains you might have gotten earlier, it discourages you. Self-esteem can play a factor, especially if someone is overweight and wants to work out and lose weight, but then they get self-conscious about their body, especially seeing people who are in-shape working out and not as many people who look like them working out. Lack of variety in the gym--some are heavily strength-based, some don't have enough equipment or the proper equipment.
These are just some common reasons I've heard or I've experienced that I could think of right off the top of my head. Hope that's enough to work with!