Linux on a modern PC hardware is a very capable router and can achieve the same routing performance as Cisco 72**. But it never gained the same popularity as a router as it did as a server.
One reason is probably administrative: in a big IT department different teams are attending to connectivity and to services. This is natural work division but it has its drawbacks. The networking team would consider Linux routers as a project for the servers team (after all, it is Linux), and the servers team would rightfully think otherwise (it is, after all, networking). Usually both stay polite and mind their own business.