Here's what I've observed in my little corner of the industry: The bottom line is that usually it is naturally a matter of profitability. If the airplane operates nearly full on a route then most carriers will increase frequency on the route to make things more convienient for the passenger (particularly business people). Some are seasonal (ski areas in the winter for example) and some are steady all year (industrial centers). If a route is found to be unprofitable, airlines differ on how quickly they will bail-out. Southwest is known for not wasting money for long on unprofitable routes. Then for reasons unknown to me, one airline operated three (almost empty) 727's into Monterey, CA every day for years and years before giving up. Also, there is the strange subject of "essential air service" which was the result of legislation in the US. This means that the government wants air service into many smaller areas to promote economic growth. The government will subsidise airline ops on those routes to allow profitability for the airline even though loads are very light. Sometimes airlines will operate flights just to provide a linkup to another airline with whom they have a marketing agreement/codeshare. Keep in mind that airlines have operational needs like flowing aircraft through maintenance bases for required checks, which can be of much influence. Lastly, hubs like SFO have long flow control delays when the weather goes below visual minimums. This causes airlines to try to avoid these troblesome airports, or to schedule longer ground times to have some hope of an on-time departure.
Anyways, that's all I can figure out looking through the cockpit windshield! Hope that helps and good luck