This had been a big topic even in the old forum. Here is the answer in two parts:
Learning from past experience with Windows CE, Microsoft is working with IC vendors directly to perform the port of Windows to processors. Some vendors have been active and others have not. NXP has been the most active in getting Windows running on their ARM processors. BroadCom has had little interest in Windows running on Raspberry Pi, thus the lack of support. Having ran Windows IoT Core on Raspberry PI 2, the performance is very slow compared to other platforms, where Linux runs perfectly fine on the Raspberry Pi
After Microsoft dropped Windows CE for big Windows, and exclaimed to the world they will never componentize big Windows again (Windows Embedded), new leadership woke up to realize that it is counter to Microsoft cloud strategy to want to connect billions of devices to the cloud and not have a small foot print Windows solution available. Windows IoT Core was sold within Microsoft to support UWP and Azure connected device. The current architecture and feature implementation of Windows IoT Core is not a popular solution. Microsoft is working to merge IoT Core and IoT Enterprise together: https://www.zdnet.com/article/microsoft-to-combine-windows-10-iot-core-and-iot-enterprise-in-2021/. If you want to learn more of this future direction, there is a virtual event coming up that will provide some details: https://www.arrow.com/arrow-services/msembedded/windows-iot-virtual.