tbf, vulcans aren't the only telepaths in the federation. WNMHGB heavily implies that there are human espers, for example, and iirc that the reason mitchell and dehner are affected is because thay're latent ones..Remember, "The Menagerie" was only halfway through the first season of TOS. IIRC, Spock had done all of two mind melds at that point, in "Dagger of the Mind" and "Return of the Archons." In "Dagger," It was painted as a highly risky procedure, and in "Archons," it was unsuccessful. It wasn't the go-to move that it would become by the second season.