Well, I think, they are, first of all, afraid to be outside their comfort zone opting for those actions they are used to doing. They might also be afraid to fail or feel insecure in the face of the unknown. Some people can just be afraid of changes and have a fear to lose something they already have. Others might not want to try something new 'cause they don't know what it's like and are worried that the action will produce an intolerable immediate outcome. I guess these are the main reasons.