The reason a leaked password is devastating is that passwords are reused, both on the site itself (people will select variants of their old passwords when changes are demanded, and eventually rotate back to the original), and, more importantly, on other sites. This means that a compromise of a database of passwords endangers not just that site but many other unrelated sites.

TOTP and U2F keys aren't cross-site, and they don't occupy any space in the user's brain. They can be treated as ephemeral and effaced and regenerated. So we're not generally that alarmed at the prospect of them appearing on Pastebin, modulo of course the implications that has for the rest of our security.

Almost every realistic scenario in which TOTP/U2F keys are leaked implies that all serverside security has been lost anyways, so protecting the keys themselves is a secondary concern. Any reasonable site operator would invalidate all stored keys after such a compromise.

By the way: salting hashes doesn't really protect against password recovery. In modern systems, it's "stretching" that does the real work. You should be using a modern password hash like bcrypt or scrypt, not simply randomizing a standard cryptographic hash.