Yes, there are a couple of reasons:
1. The Farseer Physics Engine projects in the solution does actually not contain any files, only links to files. Each time we add a new file to the project, we need to copy that file link into body .net vanilia, silverlight and XNA. If we had a Xbox360 and
WP7 project in there too, we need to copy the file to those too.
2. Whenever I release a new version of FPE, I need to pack everything together, clean up the directories, unpack them, build them, test them and release them. Most times I forget something and quickly re-upload the files before too many people get affected
by it. If I add Xbox360 and WP7, there are more that can go wrong.
3. Up until recently I did not have any WP7 device to test on. I do have one borrowed from Microsoft now.
4. WP7 and Xbox360 projects would have to be edited to stop deploying while testing.
5. WP7 and Xbox360 project can be auto-generated using VS2010 from the XNA project. However, this is not 100% fully automated as it has some problems with DLL references.
6. Less is better - it is easier to navigate the solution with a few projects.
Now, despite all those reasons, I will probably add a WP7 project (and Xbox360) up to the next release to make sure everything works correctly on those platforms. I've also done some extensive performance optimizations on Xbox360 recently that might cause
some trouble. - So everything has to be tested on all platforms.
I'm not sure if I will release WP7 and Xbox360 projects or a guide to convert the XNA version to those platforms. I might just release them and hope for the best.