system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What makes UNIX/Linux more stable and secure than Windows?

Caveat:  I'm looking for reasons that aren't "there are more viruses written for Windows", unless there's a reason there are more viruses for Windows that isn't Windows' huge market share.

Every time I read about Linux I see it touted as more secure and stable than Windows.  I remember the ISS recently (-ish) upgraded from Windows to Linux for stability reasons.  How are the OS's built differently that gives rise to these differences?

Thanks.
gpt
In UNIX/linux, the default security model has always been that normal users cannot mess with the system configuration. In windows, the default security model has long been that any user could do anything they desired to the system, and had access to all critical system configuration and other resources. Admittedly, this has  changed in recent Windows versions, and the reputation continues.

A second point of stability is that in UNIX/linux, the operating system and the graphical desktop are two completely separated systems. To the operating system, X (the window system of linux) is just another application running in user space. If X behaves badly, a sysadmin can kill it and restart it without having to reboot the system. In Windows, the graphical shell is integrated very closely with the operating system and run with admin privilege. This means that anything going wrong in the graphical shell has the ability to bring the whole system to a screeching halt, requiring a reboot.

Yet another consideration. UNIX/linux has traditionally been developed as a community effort (open source) and usually by people who knew what they were doing, because the roots of both the linux kernel and GNU lie in academia where the brightest minds competed for karma and not for money. Linux being open source means that the community as a whole is well equipped to research any bugs or strange behaviour encountered in the usage of it. In contrast, Windows has been developed traditionally by wage slaves and with the specific purpose to make money from it. It is closed source, and many of the techniques and protocols it uses are proprietary, meaning that if something goes wrong, you are unable to investigate and must rely on Microsoft to fix things. In general, the solutions and enhancements for Windows are decided from a cost/gain point of view, while solutions and enhancements in UNIX/linux tend to be chosen on the basis of technical excellence.