"The majority of Lumia phones, from our most advanced to the most affordable, can be upgraded to Windows 10. That said, for this technical preview, we need to start with a small subset of devices in order to isolate OS issues from hardware or board support package issues so we can stabilize the platform. This is a normal part of the engineering process, but you would not have seen it in the past because we haven't done a public preview before—so those builds were Microsoft only. (In the first 4-6 months of development, our team would typically focus on only one specific device—then we'd broaden.)"
"We have a feature that will be coming soon called "partition stitching" which will allow us to adjust the OS partition dynamically to create room for the install process to be able to update the OS in-place. Until this comes in, we needed devices which were configured by mobile operators with sufficiently sized OS partitions to allow the in-place upgrade, and many of the bigger phones have very tight OS partitions. Note that this doesn't mean that Windows 10 will take more disk space than Windows Phone 8.1, it's just a function of the upgrade process at this point. Once the partition stitching feature is completed, many more devices will be supported."
