For lots of reasons. Simply having a policy in place whilst necessary, is not sufficient to create an inclusive organisation. To start you need to have a clear vision of what you are aiming for and how it will be manifest in your organisation. Importantly to be effective, buy in is vital from senior management and well as a well planned implementation process.
Change only occurs when the decision makers implementing the process make more inclusive decisions. The initiative must encourage this by making them acountable and shifting their thinking and behaviour. Accountability should be built into the inclusion strategy as part of the initial design. The design phase can be structured to help achieve this from the beginning. All the stakeholders in the process must choose to accept accountability for things with which they agree and that help them attain their key objectives.