Alignment. I think this way:
The following groups would not want to be separated:
Mon/Bos
Det/Tor/Buf/Ott
NYR/NYI/NJD
Pit/Phi
Leaving: Was, Car, Cmb, and the 2 teams in Florida.
Now, you put that together so you have 2 groups of 8.....
Columbus fits ok with Pit/Phi, and then you have Was/Car and Flo/TB. No matter which way you do that, you have groups working across each other on the map.
The thing that really messes up the possibility of a clean Eastern alignment is actually the Montreal/Boston thing. If not for that:
Great Lakes: Montreal, Ott, Tor, Buf, Det, Phi, Pit, Cmb
Atlantic: Bos, NYR, NYI, NJD, Was, Car, Tb, Flo
And, economically, that would work just fine.
Working from that as a start, you could swap Bos for Phi, and leave the names the same, and it also works, but Pit and Phil were strongly against anything that split them up.
So, the result is that it ends up the way it is, because Columbus, Carolina, and Washington want to be as close as possible to their division-mates, and so Tampa and Florida end up out in the cold.