I don't think it even gets down to good vs. lousy salespeople. On a local level, it is possible to put some boots on the ground and sell lib talk, oldies, ethnic and the whole nine yards. However, the commitment to these formats on a national level is less than impressive. The Clear Channels of the world see that national buys who often pay top dollar don't flock to many of these formats because they don't fit the profile of their target audience. They sometimes don't even get any numbers, which can take localization, time and money. When the budgets are even being slashed by the big chains for their dominant station A, then how much local commitment will station B get?
Maybe liberal talk would get some extra time to establish itself in Brattleboro, VT, but larger market owners don't give questionable returns much of a lifespan. With that kind of timeline, some dedicated local salespeople don't get much of a chance to warm mom and pop stores up to the new flavor of the month to make enough of a difference. By the way, cracking the whip doesn't improve subpar sales any more than it does any other form of productivity.
It is a shame, from a listener and radio geek standpoint, that real alternatives so often get shot down before getting a chance. The simple fact is that a big AM (station A), often the only AM left in town with over a 3 share, is usually making the money targeting conservative, middle-aged males who spend more money than some other potential audiences. That type of conservative talk can be sold in combo with a recognizable name like ESPN on an also-ran sister (station B). If both liberal talk and sports get a 1 share max, then sports would be the easier sell when you're already getting the bigger bucks selling Rush.
Maybe the day will come when more companies stop treating formats as flavor of the month as long as their primary stations make money. It takes commitment and consistency in sales and programming.