Default.
Unlike the east, where there's a city every 50 or 75 miles big enough to support a full complement of TV stations and thus become its own market, you go 200 miles out from Phoenix in every direction except southeast and you get...not much.
In 1960, the population of Mohave County was barely 8,000. You can't sustain a TV station in a "market" that size, so if people wanted TV in a place like Kingman back then, they erected microwave links and translators to bring in stations from the nearest TV city, and that was Phoenix. (And sure enough, the 1961-62 Broadcasting Yearbook shows Kingman translators on 70, 76 and 82, relaying KVAR, KTVK and KOOL-TV from Phoenix.)
The viewing patterns established back then eventually determined where the lines were drawn for Arbitron's Areas of Dominant Influence (ADI) - and those ADIs eventually determined must-carry for cable. Every county is part of a market, and if Mohave County doesn't go with Phoenix, where does it go? (The same was true in other huge western states with spread-out populations - all of Utah is in the Salt Lake City ADI, nearly all of New Mexico is in the Albuquerque ADI, and all of Nevada is divided between just two markets, Reno and Las Vegas.)
What nobody could have expected back then was that Phoenix would grow so much that it would actually be lucrative eventually to build a station in Kingman (Mohave County's population has exploded from 8,000 in 1960 to 200,000 today), or that the rules could someday be used to try to leverage that Kingman station on cable in Phoenix. Call it the law of unintended consequences...