AnswerThe FCC says we have to do it this way. Period.
I am not sure how the total configuration for each station is finally decided. But there is a specific line of demarcation, possibly the Mississippi River, that governs part of this question.
All stations to the east of the line must have a "W" in its call letters (the station's public name) as in WNBC in New York City (NEW YORK CITY???!!!!) and represents a specific frequency on the AM band (Alternating Modulation} or the FM band (Frequency Modulation) over whichever band the station has a license to broadcast from the FCC,
For stations to the west of the line, the regs are the same with the exception that their call letters must contain the letter "K', as in KCBS (San Francisco or Los Angeles). I assume no station can have both. However, the required letter does not have to appear as the first letter. For example, CKLW is a major radio station in Detroit.
[For us old-timers: CKLW is the originating station that first wrote, produced and broadcast the radio series "The Adventure of the Lone Ranger" in the late 1930s. Its popularity caused its eventual national syndication. It ran 3 times a week in the evening until about 1954 when it became a TV series. And yes there was a time before TV! The imagination processes ran wild envisioning the action that was occurring in each show. It held many of us youngsters (youngsters then, at least} spellbound for the entire 30 minutes.]
Although they existed, prior to the 1960s, FM stations were not all that popular (weak broadcasting power and many, if not all, could only broadcast from during daylight hours so not to interfere with the major national stations signal in non-daylight hours. the signals carry much farther at night}.
They really came into prominence in the 60s, incidentally coinciding with the huge popularity that had grown for rock 'n roll music and it counterpart mandatory component, the "disk jockey". Eventually, they were given increased power and allowed to broadcast 24/7. They really did push the AM stations into the background, and rather quickly. This resulted in vacancies on the AM band.
The result allowed smaller local stations with a small radius of listeners to come into existence. This was a positive for small and tiny communities who could now have their own local station dedicated to local needs. These "little guys" could at now have their own local community station, serving local citizens, business and issues. And likely, a greater sense of community pride.
Additionally, it paved the way for special interest groups who wanted a station that had an even more rifled focus such as religious, political or ethnic platforms or focused music. Naturally there was a growth of foreign language stations to accommodate some of these needs. Tangentially, this allowed huge numbers of those whose second language was English, or spoke no English, to hear music, news and discussions in their native language.
Scan your AM dial today and I think that what will be found is mostly news, weather, sports, talk show hosts with call-in questions about topical issues, religious services, religious/spiritual/political discussions or individuals putting forth their own views on just about anything.
I hope this answers your question and likely more than you ever wanted to know about this. But there is no extra charge for the history lesson. I hope you were not bored to death by this.
I believe the first author's discussion of CKLW is incorrect. It begins with a "C" because it is a Canadian-registered station (Windsor, ON, across the river from Detroit, MI. Also, why are there stations east of the Mississippi that start with a "K" (KDKA in Pittsburgh)?