They also used to get paid by the segment, not the hour, so there was an incentive to get the turns done quickly.
The fact that they have "all hands on deck" workrules when it comes to turning an airplane around can't be underestimated, either. The FA's are expected to pitch in, so they do a lot of pre-landing cabin prep you don't normally see on other airlines unless the FA's are also expected to clean at the destination (i.e. the C bow).
"Avoiding hubs" is an urban myth. Yes, they avoid ATL, MSP, DFW and the slot controlled airports. But they're pretty firmly entrenched in LAS, PHX, SLC, PHL, DEN, LAX, MCO, and increasingly, SFO, all of which are hubs, and some of which are congested. Even MDW gets a little ugly at times.