Today’s roll-out to a subset of Windows 8.1 devices is based on the customer experience we have heard from you, approvals from device manufacturers, and validation by mobile operators. We also have heard through feedback from Windows Insiders that many older Windows Phone devices did not meet our expectations for quality in experience with Windows 10 preview builds, and that feedback helped us choose which devices would receive the update to Windows 10.
As we stated previously, devices which are eligible to receive Windows 10 will also be able to get preview builds from the Development branch. We’ve already delivered a few builds to the devices which shipped with Windows 10, including the Lumia 550, 650, 950, 950 XL, Xiaomi Mi4, and Alcatel OneTouch Fierce XL. We have updated the list of devices that will be capable of receiving updates as part of the Windows Insider Program going forward. Devices on this list that opt in to the Fast or Slow rings, will begin to receive new Windows 10 Insider Preview builds from the Development Branch in the coming weeks. Windows Insiders who opt in to the Release Preview ring will receive app and servicing updates from the Current Branch earlier than other customers, but will not receive in development Windows 10 Insider Preview builds.
Please check out this forum post with more details on the Windows Insider Program device list and frequently asked questions.