Why is Southwest Airlines Profitable?

Most airlines teeter on the brink of bankruptcy.  I understand that some airlines now charge for carry-on bags and one even installed a coin device to charge for restroom visits.  I am no airlines economist but this nickle and dime revenue stream is not going to solve their long-term and structural financial issues.  Rather, I believe it will achieve quite the opposite and result in losing passengers to other airlines.

Now, let's take Southwest Airlines, one of few profitable airlines in the US.

The father of two teenage girls watched as each struggled to place their one suitcase on the scale. He knew they would be over the limit. The airline representative advised the father that each bag exceeded the limit and he would need to pay $100. The attendant also pointed out that his daughters had only one of their 3-bag limit per person. The attendant suggested that the father buy a two Southwest Airlines duffle bags for $25 a piece. His daughters could transfer items from the over-the-limit bags into the duffle bag, save $50, and have a nice duffle bag to use on future trips. The results: the daughters love their duffle bags and use them for trips (free advertising); the father saved $50 (happy customer); and, the father tells this story all the time to friends and colleagues (loyal customer, residual business, company goodwill and potential future customers).

It seems that most airlines just don't get it.  They are out of touch with their customers.  Why don't they emulate the pactices of successful airlines, like Southwest?