
Has not happened because SUN did want it to happen. Want to have your runtime built in in a mobile? Ask the manufacturer. Of course, if the phone has any special features that make it special, different or simply better than the competition, the phone manufacturer will want you to have an API to take advantage of them. If not, you can take your runtime elsewhere, thank you.
In as much as MS is used to blackmail hardware manufacturers, this will not work with the mobile phone ones. They've learned the lesson from PC/PDAs, and know for sure that if they buy into the MS runtime all their phones will look and behave the same in the long term, andit will be MS who will determine the specs of the new generation phones.
Sorry, Microsoft, unless you make it both free (no fees) and open (source code available, anyone can modify it) no mobile manufacturer is going to take you seriously.