As natch said, lack of documentation. This lack of documentation prevents others from developing their own drivers etc.
>why so many manufacturers do not make "open" hardware
Multiple reasons. The first four of which I am confident that often play a large role, and the two last of which are more speculation.
1. Companies might feel that open hardware would put them at a disadvantage because others could copy them.
2. Company might have licensed portions of the hardware from other companies and therefore must adhere to agreements about not sharing information about the licensed hardware with others.
3. Maintaining an open project is a lot of work for a company. Everything from ensuring that the whole process is repeatable, to having a team of lawyers approve everything for release.
4. There is no incentive. Users buy their phones either way, even if everything is closed and proprietary.
5. Regulations regarding wireless communications hardware might apply?
6. Company might be using software tools in the hardware development process that they can't share freely so even if they did share the files others still wouldn't be able to use the files for anything?