I believe this is primarily because the development environment is really simple for this class of software. Platforms such as .NET, COM, MFC and VB have been around a long time and it's easy to find programmers. It also doesn't hurt that the desktops running Windows are incredibly inexpensive and easy to procure and run Microsoft Windows.

But targeting the end users of healthcare (e.g. patients)... that's something else entirely which only recently has started to have any traction.