Why would anyone fly any other airline besides Southwest? [closed]

Well for one, in October this year, Southwest ranked LAST in on-time arrivals. Certainly that's a reason for many - lots of people I know don't fly Tiger in Australia for this reason.

But often the primary reason is loyalty and frequent flyer programs. If I have a major alliance membership - Star Alliance, OneWorld, for example, I like to look for airlines in that alliance for points. Those with points or status will look to use their points on similar airlines. Sure, Southwest has Rapid Rewards, but is not a member of a major airline alliance.

Two things you mention in your question are free checked bags and seat selection. However, you can get those too on traditional carriers for not too much effort.

On the free checked bags front, most of the carriers seem to offer a credit card that includes the first checked bag for free. With all the main frequent flier programs, higher status tiers include checked bags too. So, for someone who flies with a carrier a fair bit (enough to either get status or want their credit card), the checked bags thing is already covered.

On the assigned seating front, again that's something that status will normally get you. Depending on your frequent traveller status, it may be available from booking, or maybe only some time later, and may or may not cover "premium" seats (with more leg room). However, again, if you fly quite a bit with one airline / one alliance, you can easily get that covered.

So, for those who care about those things, Southwest offering them may make a difference to an occasional flier, but more frequent fliers are likely to already have them on their preferred airline. If you look at the passenger numbers of carriers like Ryanair and Wizzair, some customers are happy to do without either to save a few quid, so for those people offering them may not make much difference either way.

As with many things like this, you first need to work out what things do and don't matter to you personally, only then can you work out which airline / program / hotel / etc is likely to be the best fit for you. The best fit for me is very different to the best fit for my sister, which is different again to the best fit for my parents, and that's just one family! There are rarely simple globally applicable answers to this sort of thing...

A person may fly on an airline other than Southwest because their company has a contract with another carrier(s). For the contract's requirements, Southwest is not the 'best' carrier; Southwest may not even be interested in bidding on the contract.