There isn't, and never was, a benevolent dictator choosing the OS for computers in medical settings.
Instead, it's a bunch of independent-ish, for-profit software & hardware companies. Each one trying to make it cheap & easy to develop their own product, and to maximize sales. Given the dominance of MS-DOS and Windows on cheap-ish & ubiquitous PC's, starting in the early-ish 1980's, the current situation was pretty much inevitable.
To add detail for those that don't understand, the big healthcare players barely have unix teams, and the small mom and pop groups literally have desktops sitting under the receptionist desk running the shittiest software imaginable.
The big health products are built on windows because they are built by outsourced software shops and target the majority of builds which are basically the equivalent of bob's hardware store still running windows 95 on their point of sale box.
The major players that took over this space for the big players had to migrate from this, so they still targeted "wintel" platforms because the vast majority of healthcare servers are windows.
Its basically the tech equivalent of everything evolved from the width of oxen for railway.
Instead, it's a bunch of independent-ish, for-profit software & hardware companies. Each one trying to make it cheap & easy to develop their own product, and to maximize sales. Given the dominance of MS-DOS and Windows on cheap-ish & ubiquitous PC's, starting in the early-ish 1980's, the current situation was pretty much inevitable.