It's not a big mystery. Southwest flies more city-to-city routes in an effort to lessen cost. What that means is, they'll have a plane do something like Boston to Raleigh to New Orleans to Denver to (wherever). Whereas the biggest airlines do more to-and-from the hub flights (AA using DFW, United using IAH, Delta and Atlanta, etc.). Southwest's model is an efficiency in optimal conditions, but a deficiency when you need to reroute or re-plane passengers, and it also creates huge failure points if a city isn't operational as part of the chain. Add in the staffing shortages from seasonal outages, the outdated systems others have mentioned, and then the unprecedented cold hammering some of Southwest's quasi-hubs, and here we are.
Seriously though, it's not a mystery. Airline travel has both pushed and been driven toward efficient, lean operating approaches over the years. Dollar decline, costs up, and they hunt for efficiencies wherever they can find them. Anyway, super-lean systems or "just in time" systems are often fragile, because redundancies and backup systems are seen as a (unnecessary) cost in optimal operating environments. You could take a half-full flight all the time in the old days, but now? Full flights or the routes get closed. Southwest just got caught with its pants down. Way down.