Windows IoT Core will come to an end in a few years so there is not activity other than updates moving forward. ARM support has been added to Windows IoT Enterprise in the form of the iMX8 processor. Windows IoT Enterprise will be the main OS for embedded/IoT moving forward.
There have been many discussions on the lack of support for the Raspberry Pi. First, Windows IoT Core doesn't run very well on the Raspberry Pi versions that were supported. IoT Core running on the UpBoard runs much faster. Second, Broadcomm doesn't want to make any deals with Microsoft to have Windows support Broadcomm processors. There is some licensing issues, and neither company is on the same page. It is up to the two companies to make this happen. Don't hold your breath waiting for something to change.
If you really want to use the Raspberry Pi, then stick with Linux. If you want to use Windows, then look to something like the UpBoard or other x86 PC hardware.