It was DL and AT (by hiring/outsourcing Ryan's 320's until they could get their own 737's) that double tag teamed JB and whooped their tails. That was one of the very, very few examples of DL putting up a fight against the endless growth mode capacity dumpers back then.

So JB was faced with duking it out in ATL against not one but two airlines that planted their flags with the intent on fighting to the bitter end no matter what. JB pulled out, and reallocated those AC (and many, many, many others) to accepting DL's (as well as AA and US) gift of a free terminal and instant critical mass in BOS that allowed them go from nothing to the biggest airline in BOS history practically overnight. They are now using that base as a safe haven to launch offensives into any existing market they choose still thinking no one can or will do anything about it.

They will soon (later this year and 2015) be in for a rude awakening.