Scott Fybush is indeed the expert on this and undoubtedly will have more (and more accurate) comments on this. A couple things I can share...
A lot of the allocation of radio stations dates back to the late 1920s as the Federal Radio Commission (predecessor of the FCC) tried to bring order to a chaotic AM band. Back then, stations applied for licenses and, under the 1912 radiotelephone laws and 1922 Commerce Department regs, put their stations wherever the hell they wanted and ran whatever power they pleased or could afford on whatever schedule they could afford to run (anything from a few hours a week, up to a few pioneers in big cities running 24/7). They just picked channels that allowed them (they thought) to dodge interference from anybody else.
Needless to say, it didn't work.
So Congress created the FRC to allocate stations to specific channels, and hours of operation in many cases, so the band wouldn't become a cacophany. They gave broad powers, and the FRC used them to shut dozens of stations down completely and force others to share time on a frequency. They went on to classify stations according not only to their hours (letting some operate only sunrise to sunset, others around the clock) but their coverage area. Bigger and more spread-out population centers got bigger coverage stations, smaller and more compact towns found their stations were usually licensed for smaller coverage areas through lower power. There were four classes; wide coverage stations with 5,000 to 50,000 watts and either a monopoly on their channel or limited sharing with one or two widely spaced stations giving each other minimal interference on the channel; regional stations given 500 or 1000 watts to cover a midsized metro area, and a few hundred miles of spacing between stations on the regional channels; local coverage stations able only to cover a single city or county and given 100 or 250 watts by day, 100 watts at night; and daytimers that had to shut down completely at sunset. They allocated channels so that the middle of the band got most of the high power stations (although some were put at the bottom and top of the dial as well), the bottom end got mostly 500 or 1000 watt signals which still gave decent coverage at longer wavelengths, and the higher frequency channels were a mix of regional and local stations with just a few channels (1460 to 1490) playing host to a handful of 5,000 and 10,000 watt stations. That setup was in place by 1928 and stayed in place almost until World War II...with modifications, it stayed in place even after the band expanded from 550-1500 to 540-1600 in 1941. Stations on regional channels were allowed to raise power to 5,000 watts, either fulltime or during the daylight hours, after directional antennas came along and proved themselves in the early 1930s.
How did this affect Rochester? We got only one really high power station because only Stromberg-Carlson, which bought WHAM from the U of R, had the foresight to pick an otherwise unused channel in the middle of the band (1150 in 1928, moving to 1180 in 1941) and had the resources to equip and build out a high power transmitter for itself. There was only one other station in town (WHEC) before 1936, and Frank Gannett only wanted to use it to serve the immediate area, so he didn't try for a better allocation or more than 1000 watts of power in the pre-directional antenna days. The third station, WSAY, came along late enough that there wasn't room for more than a local channel station under the old, stricter allocation rules until after the AM band expanded in 1941, so it started as a 250 watt peanut-whistle. The directional antenna liberated it, and WHEC, after the war. But Syracuse, Buffalo and NYC had beaten Rochester to the punch when it came to claiming the best regional frequencies and (in New York's case) the clears. They all had multiple stations on the air several years before Rochester even got its second fulltime station (WHEC) in 1925.
Bottom line? Federal restrictions gradually tightened, while Rochester was slower in starting up radio facilities than other communities around it.