Our infrastructure is roughly 95% windows and 5% Linux or Unix. Simply put: the requirements of the software/systems that the hospital requires to function properly is what dictates the OS.
We have a couple of major systems running on a handful of AIX Unix boxes and several dozen other systems that run RHEL, Cent, and Ubuntu. Not including hypervisors, the rest of our infrastructure is windows based and ALL of our workstations are windows.
Every app is unique, and annoyingly there is no consistency within all of a single companies applications. For example, I’m working on a GE Carescape upgrade which uses CentOS 7 but GE Time and Attendance uses Windows Server.