" *AND* Linux. Not just Red Hat either."
Too many distros and too many subtle differences across them. Plus, ugly and limited widget sets for the GUIs, compared to macOS and Windows. Linux is a system still very unfriendly for GUI applications, and also it lacks good development tools for them.
Linux really needs a complete, well designed GUI system and libraries, managed like the kernel. X is too limited, and you can't let the interface elements do be developed by KDE or Gnome for their desktops - also being forced often to use libraries designed to be cross-platforms means less perfomant and uglier GUIs. Qt, wxWidgets & C. are not adequate.
Bad ugly fonts because they need to be "free" is also a big issue. License good fonts. If the graphic cards needs closed source drivers, support them - users want great graphic performance, only a subset (the 4% actual users, probably), has its heads stubbornly buried into the "free" sand. Many companies won't give their IP away, learn to live with it.
GUI uniformity across application and across systems, plus performance, is very important for users. It's what the loot at and feel as soon as they start to use an application. Otherwise even the uglier GUI of Windows 10 will look better and faster.