As noted, it's primarily about revenue maximization and airlines charging higher fares for dates/flights where they are either seeing or anticipating higher demand (by limiting or completely zeroing out inventory in lower fare buckets to force people into purchasing higher priced fares in higher classes). aa.com and delta.com websites don't even allow you to specify a particular fare class when booking flights. Things get even more complicated with domestic F and Basic Ecomony fares as there are actually dual inventory checks. As an example, a HFA7OFFM fare basis fare is an FC fare (second to last letter indicates cabin) that books into Z booking class, but has an H class fare basis. So there must be both Z FC and H coach bucket inventory availability to book that fare. If either one is zero, the fare is not bookable and you will be forced to book a higher class fare to get FC on flight.

Additionally, there are common restrictions like advance purchase requirements for cheaper fares. Domestically, many of the cheapest fares have 3 week advance purchase requirement and are no longer available for purchase within that window even if there is bucket availability in their class buckets (2 week, 10 day, 7 day, and 3 day APs are also commonly found as you go from cheapest to more expensive fare filings). Cheaper fares having roundtrip booking (and minimum stay) requirements are still quite common internationally. Even domestically they can still be found on certain legacy routes where there is little or no LCC competition (for example, out of hostage hubs like DTW on DL).