This issue is uniquely Southwest. They try to straddle a line between a major airline and a budget carrier. Customers used to benefit from the best of both of those worlds, but this particular problem highlights the worst of both worlds.
They run a huge flight network but try to depress costs aggressively in dangerous ways. Two such ways are their refusal to invest in 21st century IT and their refusal to sign interline agreements with other airlines.
When you fly American, Delta, or United (for example) and they can't get you to your destination, they have agreements with the other two airlines to put you on a competing flight when necessary. Many years ago, I was stranded by Delta after a huge storm went through Atlanta; their interline agreement still got me home the next day on a US Airways flight. Budget carriers like Spirit, Allegiant, and (you guessed it) Southwest don't sign those agreements.
As a fairly frequent traveler, I hate the inability to reserve my seat in advance and I also don't like Southwest's reliance on secondary airports (e.g. OAK instead of SFO). Their prices are also often no better than the Big Three. They used to offer a mainline product at a budget price; in many respects they now provide a budget product at a mainline price.
They stepped on their **** this time.