I'm guessing that something about ssh makes it difficult to implement the block oriented (vs stream) comms that protocols like 5250 and 3270 need. Unlike the unix world, they can interact with the terminal without sending things to the host, and there's support for something sort of like html forms, tab traversal, etc. They fill out fields and press a send key. You can't, for example, use a normal telnet client. There's tn3270, tn5250, etc, that layer the extra stuff needed.

You can find "sortof ssh" solutions for AS/400's and z/OS, but they create a tunnel and it's still telnet underneath. Or solutions that are ssh, but are interacting with a linux partition, or posix subsystem, and can't do the 3270/5250 stuff.