Because the otherwise realistic novels of the early- to mid-twentieth century that feature telepathy always portray it as an intermittent force the characters can't fully control that comes only in times of great need, whereas in Touch Not the Cat the heroine and hero basically have their own personal IM system, like something out of Willo Davis Roberts' children's fantasy The Girl with the Silver Eyes or one of Lois Duncan's supernatural thrillers.

I know there was a lot of interest in ESP, clairvoyance, etc. etc. in the 1970s, but I'd need to see some pretty strong evidence of mainstream belief that some people can speak to each other telepathically as if they were talking on the phone to accept that this was seen as a realistic romantic suspense novel rather than romantic suspense novel with a fun supernatural twist.