Cisco IOS has been around for about 20 years. They've had time to get it right.
Linux has been around almost but not quite that long too (it came out in 1993 as I recall). They've also had time to get that right, too.
The majority of webservers run Linux connected to Cisco routers.
AT&T, of course, created Unix.
Linux is very reliable as a router, assuming the hardware is good. There are ways to do it without needing a hard drive, which removes a failure point.
