What motivates leaders in cuba, china, and north korea to prevent their citizens from freely accessing the internet?

The fear of its people gaining enough knowledge that can push their people to action and decide to choose another form of government.

The internet has vast information around the globe coming from past and present information as well as future ideas. For these reasons, it can be a driving force for the people to change their ways and encourage others to oppose their own government. This is far from what their government wanted which is to maintain the status quo.