When we sent out the first build of Windows 10 Technical Preview for phones on February 12th, the list of phones was very limited. As I wrote in the accompanying blog post, the reason for this is that we had to select from the set of phones that had sufficient system partition sizes configured by the manufacturer. This was necessary for us to be able to in-place upgrade the phones. I also talked about a feature we were working on called “partition stitching” that would allow us to re-size the system partition dynamically, allowing us to do the upgrade on a larger set of phones.

 

http://blogs.windows.com/bloggingwindows/2015/03/27/a-quick-update-on-windows-10-builds-for-phones-for-windows-insiders/