Windows 10 Mobile Update For Older Lumias Pushed To 2016
 

It's not made explicitly clear why the update has been delayed, as the OS is already available on Microsoft's recently launched Lumia 950 and 950 XL. From my experience with the beta/developer releases of the OS that Microsoft has made available, it's entirely possible that they still need to work on ironing out bugs and improving performance. Many reviews of the new Lumia phones have a similar sentiment, and with many of the older Lumia devices running less capable hardware from Qualcomm's Snapdragon 400 series than the 950 and 950 XL, it wouldn't be such a bad thing to have the update delayed to ensure it doesn't cripple performance on those phones.

http://www.anandtech.com/show/9865/w...pushed-to-2016