Auditing: using Windows plus AV plus malware protection means you demonstrate compliance faster than trying to prove your particular version on Linux is secure. Hospitals have to demonstrate compliance in very short timeframes and every second counts. If you fail to achieve this, some or all of your units can be closed.

Dependency chains: many pieces of kit either only have drivers on windows or work much better on Windows. You are at the mercy of the least OS diverse piece of kit. Label printers are notorious for this as an e.g.

Staffing: Many of your staff know how to do their jobs excellently, but will struggle with tech. You need them to be able assume a look and feel, because you dont want them fighting UX differences when every second counts. Their stress level is roughly equiv. to their worst 10 seconds of their day. And staff will quit or strike over UX. Even UI colour changes due to virtualization down scaling have triggered strife.

Change Mgmt: Hospitals are conservative and rarely push the envelope. We are seeing a major shift at the moment in key areas (EMR) but this still happening slowly. No one is interested in increasing their risk just because Linux exists and has Win64 compatability. There is literally no driver for change away from windows.