As Southwest expands its route network, it needs to allocate their available aircraft to routes believed to bring in the most revenue - not necessarily where there will be the most demand for seats. A full plane does not mean that the flight is making a lot of $$$.
When Southwest has competition from other carriers in a particular market or routes, the fares will tend to be more competitive; it generally works that way with most airlines. In smaller markets, where there is little or no competition, the fares tend to trend a bit higher.
Also, since the economy has been in recovery and more businesses and individuals have been traveling, there are very few flights with lots of unsold seats. Again, it is supply and demand. The airlines no longer need to entice customers with extremely low fares, as when the economy was not doing well.
When Southwest has competition from other carriers in a particular market or routes, the fares will tend to be more competitive; it generally works that way with most airlines. In smaller markets, where there is little or no competition, the fares tend to trend a bit higher.
Also, since the economy has been in recovery and more businesses and individuals have been traveling, there are very few flights with lots of unsold seats. Again, it is supply and demand. The airlines no longer need to entice customers with extremely low fares, as when the economy was not doing well.