Slot squatting is a big part of this, but there is a much bigger picture here.

The pilot shortage is getting really acute now, and it's 50% or so longer to go from these cities to DTW vs NYC (double for ITH/BGM). I know it's not as simple that just because a flight is half the distance does not mean it takes half the time, but there is an hours saving in there.

From a network standpoint, these cities were all getting once daily service (normally departing the outstation around 5 pm) - not the most conducive to service. In ITH, you've got UA running twice a day to EWR, SCE you've got multiple frequencies on UA and AA to PHL/EWR/ORD with multiple frequencies. Now you can offer 2x/day service (much more appealing for travelers to use these airports now versus driving to SYR/MDT) and a lot of the traffic is staying on the East Coast (hint - a lot of it is going to Florida). Add in the fact that at LGA, you've had slots open up for a combination of quite a few things - New York City is still under 50% office space being used, which means business travel is down, so a market like LGA-BHM that was 2x/day pre-Covid is down to 1x/day, plus a lot of upgauging going on with the A220 now being used quite a bit (45% capacity increase over a 175/CR9) and a lot of A321s taking over from 738s/320s (especially on Florida routes) - that's a 20% increase in capacity there, so you've got increased seats to fill. Layer in that Delta has a brand new terminal that it wants to start recouping its investment and it all adds up to making it much more commercially appealing to Delta as a company to route more traffic over LaGuardia vs Detroit. As I said on another site, yeah it's not appealing that you can't go on Delta from these cities to California without switching airports or making an additional stop, but the truth is Delta wasn't really getting that traffic as it is today, and if they were, it was either you were a Delta loyalist that would tolerate the inconvenient schedule, or you were spill traffic from more attractive UA/AA schedules. Going twice a day service and offering multiple convenient options to where people want to travel (which is a lot up and down the East Coast) makes Delta a much stronger carrier in these markets.