There’s a couple of things in play here:
DL is by far the dominant carrier at JAX and ATL is the primary gateway where they connect JAX to the rest of their network. In TPA, FLL, and MIA, DL has lower market share and those stations are better connected to other DL hubs than JAX is.
JAX has a large catchment area and is more business oriented than other Florida cities, this means that schedule, and thus frequency, is important. JAX also does not leak to other airports as the only reasonably close airport is DAB, which has inferior service and higher fares. The DAB market instead leaks to JAX and MCO. Since there are no airports of any size in southeast Georgia, JAX draws from that area too.
JAX-ATL is much shorter than JAX to any other DL hub. With fuel expensive, airlines try to funnel traffic from outstations to the closest hub and reduce overflying to further hubs. A simple example of this is that when fuel went up in 2008, United cut frequency on ORD-LAS and added frequency on DEN-LAS - offering the same amount of seats to the market but hopefully having to haul passengers fewer miles and using less fuel. Stations that lie in between multiple hubs often see more balanced service, for example CMH, which is about the same size as JAX, has a good mix of flights to ATL, DTW, LGA, and MSP