So F9 has one DEN-TPA flight, whereas WN has two. F9's tickets are a penny less than WN's on this nonstop, which occurs around the same time as WN's latest nonstop/

There are also multiple connection opportunities on some days DEN-XXX-TPA, where XXX can be DSM or MKE. MKE is expensive but has a quick connection...quick enough to beat out WN's one stop "quick turn" option. The problem here is that WN offers a nonstop DEN-TPA that arrives later and gets to TPA earlier than F9, for a lower price. But if you want a flight that gets to TPA from DEN around 6-7pm on F9, you've got one or two (or zero, depending on the day) options.

As usual though, Classic Plus on any F9 flight is cheaper than AT on WN.

Now add in three connecting flights from Airtran that are cheaper (by $20) than both F9 and WN, two of which are shorter than either F9 or WN connections. Plus a couple of direct UA flights for $17 more expensive than what F9 and WN are selling...the route must be a reasonably strong market.