Most major change projects fail. About 70% of them in fact!
It is usually because there are conscious and unconscious ways in which a system will resist change. When there is change, people can feel a sense of loss. There is also no guarantee that change will affect things positively, and this risk can cause trepidation.
So in any system there will be some desire to change and some fear and resistance. These two positions work in opposition to each other, like forces, in order to try and keep things the same.
To make a change you have to really understand the drivers and resistors to change. A Force Field Analysis can help you understand these, and you can then take steps to make the drivers more powerful than the resistors.