There are a couple of things to note here...
1) HKDL is now 15 years old and pre-dates any good relationship between the Chinese government and Disney. Hong Kong was the only option at that time for getting anywhere near the Chinese market and I believe that if it were to be built today Hong Kong would not be considered.
2) The main reason is economic. Shanghai for example has close to 27million residents to compare with London's 9m and New York's 8.5m people. China is going through its own economic revolution where wages are rising along with standards of living, the middle class in China is growing ,and will continue to do so for a good time to come. That money is getting spent on leisure activities and the brands see an opportunity for growth in a growing market.
3) Government policy is changing. China was always closed to "western" brands and insulated against the rest of the world. As the Government policies change they become more accessible and open to trade, tourists and foreign investment. They don't necessarily need overseas tourists to have a population to visit the park but it helps that Disney, Universal etc can bring in workers from their home countries to oversee projects.
I hope that helps, there is a lot more complexity that that but that should give you an idea of what is happening in the background.