I'd guess it didn't choose that route because of:

-intense competition (Southwest fending off Spirit) down at BWI, for BWI-FLL. Southwest is selling one-way tickets next week for just $133-4 (3 days next week), $102 (1 day) and $90 (2 days). That's generally unseen of Southwest to offer such low fares for a near 1000 mile route just a week away. The local ILG crowd might flock down to BWI anyways.

From NJ, there are the F9 flights from TTN, Spirit from ACY and US which hasn't increased fares on PHL-FLL, and Southwest's seasonal service.

From the PA suburbs, it's whatever is there at PHL and TTN. ACY and BWI are somewhat a hike but some use them still. But, there is still good level of competition from PHL alone, and likelihood that either Spirit or JetBlue adds PHL-FLL when Southwest ends the seasonal service in March 2015.

-the seasonal nature of FLL, RSW and TPA, where demand is great during December to Spring Break, but tapers off after that.

-Frontier preferring flying routes like 400-700 miles (rather than longer ones) to get more destinations in from a focus city.

My guess is ILG-RDU would be a good choice. If offered during the evening, it wouldn't overlap with TTN-RDU's early morning runs.

However, I'm not sure what direction Frontier wants to go with ILG since nothing new has been announced. It may go in a truly more Allegiant style mode. Atlanta is farely new, and hopefully will be extended through the winter and do well. Atleast it's giving ATL a longer run out of ILG than Spirit offers out of ACY, where Spirit runs it as seasonal only, ends it in the Fall. Spirit has never even tried ACY-ATL for a full year run. Maybe Spirit feels that ACY-FLL is so profitable, it's not even worth downsizing it during peak season for other flights. Spirit can offer connections ( international) through FLL, but I wonder how many pax from NJ are flying Spirit ex ACY to fly international with a layover in FLL.