Other airlines were disrupted, but not to the same level, due to different level of dependance on said stop. The second most affected airline IIRC had Jacksonville as minor stop, and out of all impacted, only Southwest used what they call "line flow" flight planning. Which results in Southwest being the only one to end up with stuck planes that were supposed to service a many routes way further away.

Most low-cost airlines operate on a sort of "distributed hub-and-spoke" (many small hubs instead of one/few big ones), with plane going bidirectionally on a route to non-hub airport before any route switching happens. This means that so long as the "hub" isn't severely impacted your disruption is somewhat contained and you still have access to a fleet of planes&crews that can be reassigned. A pattern like this:

  H->A->H->B->H->C->H ;; H = "Hub"; A,B,C - spoke airports

Disruption at A,B, or C doesn't block unrelated flights, only disruption at H.

Southwest instead flies like this

  A->B->C->D->E->F->G

If B is in any way blocked, flights B->C, C->D, D->E, E->F, and F->G are all going to be disrupted, possibly without replacements plane/crew combinations available. If the disrupted airport has a lot of overlapping "lines" like that, the disruption quickly turns catastrophical.

Also, appears that at least some approaches were manually vectored recently at KJAX, which isn't fun if there's a shortage of controllers.