The phone manufacturer actually supplies the correct cradle design to BMW.
So if the manufacturer doesnt want to design a cradle, BMW doesnt make it.
Another problem is a number of the new phones dont have a way to connect to a external antenna. So if you put them in the armrest in a charger they will loose signal. So BMW cannot provide cradles for them.
Another problem is phones that dont have easy to connect to "docking' pins.
Like some motorola phones have a small port on the side, making it very hard to install in a cradle, and even if it may work on say a LHD car, it wont work on a RHD car. So again for these phones BMW cannot provide cradles