My response explains why DL chose to reduce service at BOS in favor of larger operations in NYC.
I'm sure you don't want to hear it, but DL has managed to create a combined hub at LGA and JFK with more than 45,000 seats per day on 440 flights that is larger than US' hubs at PHL, DCA, and PHX, AA's hubs at ORD and MIA, and comparable in size to DL's hubs at DTW and MSP, and smaller than ATL, CLT, DFW, EWR, and IAH and all 3 "hubs" at DEN. Given that DL's operations at NYC are much higher local market focused than any larger hub, DL's operations in NYC are likely also far more profitable than most of those other hubs.
When you look at the total size and location of hubs, it doesn't make a whole lot of sense to maintain a large domestic presence in a city that is a couple hundred miles north of one of the top 7 largest hubs in the US.
Given what DL has done in NYC combined with its hub locations in DTW and ATL, it makes perfect sense why BOS is now a large focus city for DL.
It also is notable that US operates NO flights from BOS-Florida any longer. In the summer of 2000, US operated 13 flights/day BOS-Florida.
DL's BOS-MCO is the only network carrier non-hub BOS-Florida service.