I don't think open sourcing it would help much at this point. It's not as if it's impossible to find a Windows phone to use, but the ecosystem is falling apart too.
The biggest problems in my mind for using a Windows phone today are really that what apps there were are going away (which open sourcing the os isn't going to help), and the browser is bad. On Windows phone 8, the browser is bad because the rendering engine of mobile IE isn't very good / workable with modern web; on Windows mobile 10, the browser is bad because Edge frequently delays responding to user input and it's uterly unusable at times. Plus or minus when the browser locks up the phone and the watchdog timer reboots it.
In part, I would imagine, because IIRC they were trying to do the same concept that Apple did about shipping one OS built for desktop/phones with different flags, so that would require opening a large swathe of the Windows source tree.
A lot of the Windows Phone OS is ported from the Windows code at the time (networking stack, libraries, UI) with fixes for multiple architectures (ARM/MIPS), power, and performance.
