Really? Oh, compared to other Windows versions...
Because it never came close to the stability of OS/400, Netware 3, AIX, Solaris or even OS/2 v2.
That is easier to achieve when your operating system only runs on your own proprietary hardware. (No mess of millions of drivers to write for one).
It worked well for years without any sysadmin touching it.
Well my mom was trained to be the "sys admin", which meant rotating backup tapes.
I saw years of uptime on those systems whereas Win2000 iirc needed a reboot for every single update of the OS, and even for applications like IIS or Exchange.
Compared to NT4 it was probably very stable, since I remember telling most clients to just shut it down Friday evening and boot it Monday morning cause the pre-SP4 NT4 could not stay up more than three weeks.
Compare that to AS/400, where we pushed updates all over the country, without warning clients, to system running in hospitals, and there never was even the slightest problem. It sounds irresponsible to do that today, but those updates just worked, all the time and all applications continued to work.
This just means security updates were never installed.
(Or you claim that all those operating systems never had kernel-level security issues which seems doubtful...)
Most were only locally connected (for example OS/2 had a Token Ring in one building). The WAN connection (for AS/400) was trusted.