Now the question becomes: What comes first the chicken or the egg? The format or the advertiser? So, how does a station choose a format or does it choose which advertiser it wants to cater to first? After all, some demos have more money than others. Some demos are more likely to spend their money than other demos. Some advertisers cater to everyone (toothpaste, toilet paper) but it is usually a specific person buying it for the household (the women). Why would a station cater to the demo of any small industry when the bread and butter is to cater to industries that serve the masses? Who spends more money? Men or Women? Why would any radio owner do a format that did not cater to the largest advertising industry, no matter the format?
Good questions.
First, it's important to note that the amount of dollars spent in any given major market in any given year is not a subject of much mystery. Budgets are drawn up well in advance. Broadcast trades and the larger financial news media report on what the likely amount of ad spending in a coming year will be. That can change based on conditions, but only by a few percent.
In my experience, stations decide on a format by looking at the amount of money being spent by advertisers in the market to attract a given demographic, then examine the stations already dividing that ad revenue. Analyses are done as to the strengths and weaknesses of the players already there (including their own...do we have the signal to cover the market adequately?). Is there an opportunity to rank highly enough to be an automatic buy in that demographic? If so, are we displacing a competitor or simply reducing the size of the slices of the pie? If the latter, is the result a number profitable for us given the way we want to run the radio station? If not, are we willing to make changes to the operating plan (voice-tracking instead of live DJs, a lower promotion budget) to make it profitable? Could additional promotion actually displace one of the competitors? What would that cost? Would the added cost reduce or perhaps eliminate the profit margin?
If the answers aren't satisfying, you move on to another demographic and perform the same analyses. Sometimes you'll find that (and this is just for illustration), while less money overall is being spent in your market on 18-34 than on 25-54, your chances are better of making more money because there are fewer competitors, or it would be easier to displace one or more of them from the format.
Again, if the answers aren't satisfying, you move on. Some stations find themselves in what might be considered "niche" broadcasting (religion, ethnic), but the numbers are better than what they could get trying to elbow into the pond where the big dogs are drinking.
If every station went after the largest ad spending category, there would not only be vast underserved yet profitable demographics, but broadcasters would continue to research, to promote, to make their stations respond to the behaviors and expectations of the audience. In time, some of them would rise above the others in the ratings, the agencies would buy the top five, seven, maybe ten of them....and the rest would starve, forcing them to go find where they could make money, which would lead to something that looks remarkably like today.