Pike's injuries and the nature of his chair crop up every so often and some interesting ideas get tossed around the threads. Someone suggested this chair was something of a "stop gap" to stabilize his condition before more sophisticated devices could be employed, like a more refined communications interface. Unfortunately, the way McCoy talks, there's no way to tie more directly into Pike's brain to allow that. And, if they could, it would "weaken" Spock's desperation to get Pike to Talos IV where he can live out his years "unfettered" by his injuries.
In short, if Pike could have talked, the "mystery" as to why Spock would seemingly go AWOL is negated. In short, it's the "McGuffin", a motivating factor for the narrative. Or, if one is cynical, it's a plot contrivance. Maybe the nature of the Delta ray damage made normal interfacing considerably more problematic.
Pulling Next Gen' into the equation, how long has the Federation had relations with Betazed? If, barely a century, there might not be too many of them around, yet (at the time of TOS). Vulcans? Well, later Spock was mind-melding with everything this side of the galactic core, but at the time of "Dagger of the Mind", Spock spoke of it in hushed tones, it being a rather private thing, almost as secretive as Pon Farr. So maybe Vulcans in general were not that willing to offer their services.