I'll admit that I haven't read carefully all that was ever written on the subject but are we sure there were no copycats?
Because I remember watching a few episode (maybe 3 out of 5) of The Hunt for the Zodiac Killer (www.imdb.com/title/tt7784996/?ref_=fn_tt_tt_27) and they actually got access to some FBI files in some of them they found that the Killer was in New York area for a moment. That moment coincide with the years for which he was "silent" in San Francisco. The letters had the signs and code he would use and there were murders that fit the patterns happening has described the usual way.
According to what was said in the TV series, the FBI still don't know if he was there for work (on his own or had been transferred there) or for travel. He was there for some time yet the info was never made public because the authorities preferred the general public to think he (the killer) was "confined" to the San Francisco area, not that he moved around the country. That way they could prevent a mass panic.