They're not exactly insecure, they're just used for different things, like networks that are already secure. You can secure existing networks pretty well against things like MitM, and then it doesn't matter if you use rsh or ssh. (part of why so many routers still use telnet is it's supposed to only be enabled on management interfaces)

Quite a few years ago I helped build the public terminal cluster for HOPE. That year we used a token ring network for all of the hosts. Nobody anywhere near the conference had any token ring gear whatsoever. It was impossible to break into the network, so you had to use existing nodes. And the nodes' adapters had their promiscuous modes disabled in hardware. That plus a hardened terminal server and an extremely limited thin client meant nobody could successfully hack the network. You could have used telnet all day and been totally secure.

Though in conference environments, the network is not the biggest risk for targeted attacks. Carefully watching your target enter their password is simpler and more effective.