Telepathic species, Vulcans included, were known in TOS.
...And were in the bogeymen category for the most part. Spock kept his abilities secret until "Dagger of the Mind", and the fact that they came as a surprise to the medically educated McCoy suggests there isn't anything about that ability in professional or general literature, either. And Miranda Jones was evicted to Vulcan for displaying signs of telepathy...
Telepaths in "The Cage" and "The Menagerie" were black hats through and through. This both gives a dramatic reason not to have a white hat telepath in the episodes, and tells a lot about the general attitudes and prejudices of 23rd century humans or Starfleeters about telepathy.
Maybe the nature of the Delta ray damage made normal interfacing considerably more problematic.
If Pike could process language in his brain in the first place, Federation science would probably have found a way to tie the processes to an output device of some sort. But if Pike suffers from brain damage specific to language processing, and can perfectly understand spoken words but cannot put together an English sentence to save (or end) his life, then not only are machine "voders" out, but telepathy might become less informative as well.
The point about Spock having no motivation to act as an interpreter for Pike here stands, though.