In the Star Trek TOS episode "The Menagerie", why didn't they have Spock perform a Mind Meld on Captain Pike instead accepting the Talosian transmissions from Talos IV as proof of what had happened?
Spock was on trial and couldn't be expected to cooperate. His testimony about the state of Pike's mind wouldn't be trustworthy, anyway. Mendez was a projection from Talos IV and so wouldn't suggest anything the Talosians didn't want nor would he allow the mind meld even if Kirk suggested it. The information stream from Talos IV was at least something all the members of the court martial could watch objectively and make up their minds about.
This is not an in-universe explanation.
The first appearance of the Vulcan mind meld was only a couple of weeks before that episode aired.
The first mind meld, as far as I can tell, appeared in Dagger of the Mind, which aired Nov. 3, 1966. The Menagerie parts I and II aired November 17 and 24, 1966. Gene Roddenberry has a writing credit for both (well, all three) episodes, but it's likely that the whole idea of Vulcan telepathy just wasn't invented in time to be used in The Menagerie.
Note that Where No Man Has Gone Before, which aired two months earlier, dealt heavily with telepathy, with Spock discussing it at some length -- but there was no mention that Spock himself was telepathic.