I don't know the answer to the question, I do know the answer to the question "Why do digital signage vendors use Windows as their host platform?" seems to be so that they can hire unskilled labor. In various embedded forums I see questions about doing digital signage on Android/Linux while exploiting the graphics acceleration of the SoC get met by stony silence (no drivers) or complex installs (blobs + release compatibilty). Its way too much work so these guys get Windows, and all the vendors have Windows drivers and are willing to let you incorporate them into your code and hiring a Windows programmer has a bigger pool to draw upon.

Combine that with customers who want to use Windows tools to configure/upload to their signs and IT folks who are "comfortable" with Windows on their network and voila, mutual lock in.