Why do most radio station call letters start with K or W?

In the United States radio call letters are assigned by the Federal Communications Commission.
There are corresponding government agencies in other countries that assign call letters in those countries.

The "W" and "K" designations for U.S. radio stations go back to the Federal Radio Act of 1934. By international treaty, other countries and areas have their own distinctive designations - "C" in Canada, "X" in Mexico, etc.

"W" was the designation for stations East of the Mississippi and "K" for stations West.
Before that rule was firmly established, some stations East took "K" calls (like KDKA, Pittsburgh) and others West took "W" calls (like WMT in Cedar Rapids, IA and WHO in Des Moines, IA).

1) "In the United States, broadcast stations have call signs between three and six characters in length, though the minimum length for new stations is four letters. An additional suffix may also be added, indicating a specific broadcast service type. Full-power stations receive four-letter call signs, while broadcast translator stations usually receive call signs with five or six characters, including two or three numbers. Generally, call signs begin with K west of the Mississippi River, and W to the east."

"New broadcasting stations are assigned call signs beginning with K, if they are west of the Mississippi River, and beginning with W if they are east of the river. Again, some early stations have been grandfathered, so there are four broadcasters with a K prefix east of the Mississippi, and a few dozen with a W on the west side. (There are more grandfathered W stations because the dividing line used to be two states farther west.) Some examples would be KDKA in Pittsburgh, Pennsylvania, KYW in Philadelphia, and WACO in Waco, Texas, which also has the distinction of being one of only three radio stations whose call sign is the same as its community of license.[6] Stations located near the Mississippi River may have either letter, depending on the precise location of their community of license and on historical contingencies. Minnesota and Louisiana are allowed to use both call letter prefixes since the Mississippi river flows through both states in addition to forming parts of their borders. Metro areas that straddle different states on both sides of the river, such as St. Louis, Memphis, New Orleans, Minneapolis/St. Paul, and the Quad Cities area of Iowa/Illinois, have stations with both call letter prefixes, due to the stations' communities of license being placed on either side of the river. [7]
The FCC allows derived call signs in the same market as a commonly-owned AM or FM without respect of the boundary, so stations may establish common branding across bands and services. One famous example was the case of the former KWK in St. Louis, which after several petitions was permitted to change the call sign of its sister FM station in Granite City, Illinois, then WWWK(FM), to KWK-FM. Later, the AM would change its call sign and the FM became KWK(FM), thereby becoming an exemplar of both categories of grandfathered stations.
The assignment of K and W prefixes applies only to stations in the broadcast radio and television services; it does not apply to weather radio, highway advisory radio, or time signal stations, even though these are all broadcasts in the usual sense of the word, nor does it apply to auxiliary licenses held by broadcast stations, such as studio-transmitter links and inter-city relay stations.
For example, the time signal stations WWV and WWVH are located in Colorado and Hawaii, respectively. (WWV originally began in Maryland and was later moved west. However, even ignoring that fact, U.S. government-owned stations are overseen by the NTIA and not the FCC, and are thus not subject to the FCC's rules on call signs; most do not have call signs at all.)
NOAA Weather Radio stations clustered between 162.4 and 162.55 MHz have call signs consisting of a K or W followed by letters, and two digits. The K and W prefixes are both used interchangeably on both sides of the Mississippi River (e.g., KHB36 in Washington, D.C. and WXK25 in El Paso, Texas).
Highway advisory radio stations scattered throughout the AM band use call signs consisting of K and W followed by two or three letters and three digits. As with weather radio, K and W calls are both used on both sides of the Mississippi River."
Source and further information:
http://en.wikipedia.org/wiki/North_American_call_sign


2) "Notice the policy was that calls for ocean-going ship stations started with a different letter than the land stations they communicated with: in the West ships received W-- calls and land stations were assigned K--, while the reverse was true in the East, with K-- ship calls and W-- land calls. (NOTE: The assignment of W and K to the United States appears to have been completely arbitrary--the letters have no particular significance. N, however, had been commonly used by the U.S. Navy since November, 1909)."
Source and further information:
http://earlyradiohistory.us/3myst.htm

Further information:
http://earlyradiohistory.us/kwtrivia.htm

Roughly speaking, station call signs east of the Mississippi River begin with W and those that are west start with K. I'm sure there are exceptions.

Stations west start with K, east coast starts with W

Stations are assigned a code and generally (with a few exceptions), stations west of the Mississippi river begin with a K and those east of the Mississippi begin with a W.

I didn't even realise that they did all start with a K or W. I can only imagine that the passing troll who downrated you took exception to the fact that you didn't identify the country you were talking about. Any, thumbs up from me.

Radio stations were assigned an intital letter for the country they broadcast. Canadia is "C". Mexico is "X". The letters were aribitrary. "W" was used first. As the US expanded "K" was added.