I'm confused. As I've posted many times before, one possible reason for using Windows in a situation like this is that while it can be a massive pain in the arse, System Center Configuration manager makes it almost as easy to manage a fleet of 1,000,000 Windows devices as it is to manage 1. This certainly includes distributing updates in a timely, and usually non-obtrusive way. Any decent MDM solution should do that. As I've stated before (and seemingly got downvoted for it), I am genuinely interested to find a decent MDM solution for Linux. I have a nice little project that I could do using PIs for work, but would need to show it's under some sort of MDM solution before they'll even look at it.
I do agree that there is no reason any advertising screen couldn't just be powered by some sort of single board computer (not necessarily a Pi, but can be), probably running some flavour of Linux.
Beyond the MDM problem above, I think in some IT departments, non Windows OSes still get some prejudice. A lot of IT technicians and system admins dismiss Linux, macOS and other OSes because they aren't Windows. Certain System Admins where I work (and these are very highly qualified people) almost seem like they would back into a corner, hissing and scratch at you like an animal if you dare suggest any OS other than Windows. This is despite the fact that both Linux and MacOS have a far superior lineage to Windows, either being a clone of (in the case of Linux) or derived from (in the case of macOS) variants of Unix.
Note: I am not knocking Windows. I primarily support macOS at work (and am thus sometime dismissed as a wierdo at work), but I also support Windows (primarily 10). I have a PC that I use at home primarily for gaming, but also some remote working, and I have a Mac Laptop that I also use for remote working, but also general messing around. I have various Pis, all running Raspbian, and I'd like to see greater use of the Pis at work, particularly for digital signage, for which we are currently using (I believe) low end Windows PCs..