What has changed is that everyone, including Southwest, has seen the need to raise fares. Also, and this goes with the reason you continue to see full planes, everyone has cut capacity successfully. There are fewer seats available; so, the people who want or have to travel are willing to pay more. There isn't a cheaper deal down the street anymore. Well, there is, but convenience and schedule issues make a lot of the cheaper deals unattractive.
For instance, my sister who lives near Birmingham, AL was looking for a flight from BHM to LAS (where her grandchildren live). She found that the most reasonable fare was on AA through DFW--one change of planes. She found a cheaper fare on Expedia or Travelocity on another airline (not WN), but it involved flying on an RJ on 1st leg from BHM (she doesn't like rjs), changing planes twice and taking something like 10-12 hours more than the AA flight.
Interestingly, though. I suggested that she non-rev on my passes BHM-DFW and DFW-BHM, then confirmed ticket DFW-LAS. When we looked at that option, we found that a confirmed space ticket BHM to LAS through DFW roundtrip was about $335. DFW-LAS roundtrip same days of travel on same flights to/from LAS was $550?????? It's pricing like that which makes the public (and me) crazy!