Seriously, that really can sum it up. Linux has made a lot of inroads, but its nowhere near the stability of operating systems that spend their time "doing work" as their first priority.
Are you grouping operating systems into "linux" and "non-linux" categories, and asserting that the "linux" operating systems are nowhere near as stable?
If you compare with Solaris, AS/400, OS/390, AIX, FreeBSD, HP/UX, and a few others - Linux loses bigtime in the uptime department.
That doesn't make Linux less useful, it just hilites the maintenance required. People (MSFT mainly) use Windows to a high degree of success, they just reboot it a lot. I have a Linux system that needs to be rebooted every 3-4 days to maintain usability, I can't do anything about it because the system itself is essentially closed.
I have a lot of customers who use Linux, the ones who don't have problems mostly run CentOS. The ubuntu users always seem to have problems, although I suspect thats usually due to it being developers running systems vs ops people. The ones who run every other Linux seem pointy headed enough not to screw themselves over.
ProTip: If you break your box regularly, you are not a good sysadmin.
Seriously, that really can sum it up. Linux has made a lot of inroads, but its nowhere near the stability of operating systems that spend their time "doing work" as their first priority.