>My only lack of understanding in this matter is why so many people aren't capable of understanding more than one Operating system.
I think it's less laziness than the efficiency of familiarity, combined with the fact that "alternative OSes" historically presented a far more different UI than has become the case with many today. Consider driving a car that had replaced the steering wheels and pedals with joysticks or something - a functionally trivial change in a modern fly-by-wire car, but your ability to maneuver the vehicle effectively is going to be considerably compromised by your unfamiliarity. Yes, 80% of the time that may not matter, but that last 20% is going to be constantly cropping up with irritating reminders of your incompetence until you have a few thousand hours of familiarity under your belt. And long after that you've achieved basic competence, the differences are liable to generate pro-active interference with each other, assuming you still drive normal vehiecls as well
While an unfamiliar operating system is generally less personally dangerous, the difficulties are still quite frustrating. Even something as ubiquitous as the file load/save dialog often presents a considerably different interface between operating systems, with many non-obvious differences in how you configure and leverage bookmarks and other non-trivial navigation aids, on top of the differences in file system organization conventions. Or heck, take the MacOS file manager with it's drop-down folder heirarchy menu from the title bar - beautiful idea, I sometimes find myself missing it on other platforms, but completely non-intuitive, and until you learn of it it's pretty much impossible to navigate up a folder heirarchy.
What makes it worse is often the differences seem to be added purely for the sake of being different - take the window min/max/close buttons on MacOSX and Ubuntu, which for some reason they decided to put on the left instead of the right which everyone has been made familiar with over the last couple decades. When switching OSes you now need to retrain your "muscle memory" on how to close windows, and if you use multiple OSes on a regular basis you probably end up momentarily confused on both. And to what end? Even if there's some grand philosophical reason to the change (and I've never heard one), the end result is that they made OS migration that much more difficult for the sake of a tiny functionality change.