Maybe because it was their first (and only) EMR system, but in the practice I worked at, all the initial infrastructure was put in by Allscripts explicitly for

Maybe because it was their first (and only) EMR system, but in the practice I worked at, all the initial infrastructure was put in by Allscripts explicitly for EMR, with all other applications being secondary. They sold the boxes, set up the Windows domain, and managed everything. Only relatively recently did they start using it as their own infrastructure, instead of just the magic to make Allscripts EHR work. In that kind of scenario, it could just as easily have been Linux.