Why the 76?
IF the rumor is true - probably because WN is very, very big on commonality. It is a core part of their cost philosophy.
Which rules out Airbus (whole different design and operating philosophy from WN's core 73s).
Among Boeings - well, any wide-body will lack a lot of parts commonality, but the 76 cockpit layout and routine will be from the same "mind" that created the 73s that WN has standardized on for decades. An easy transition for pilots, if not for mechanics.
75? - not much capacity advantage over 739s, plus its forte is narrow-body long-range, and WN (if indeed looking for big jobs), is probably more in the market for wide-body, medium range.
77/78? Again, designs focused on long-haul, and more FBW than the 73/76.
74SR (Japanese style) might fit WN's high-density domestic needs to a T in terms of a short/medium-haul wide-body. But hasn't been offered for close to 30 years. And would complicate pilot transitions with the extra 2 engines.
As to why a bigger plane at all, I'd guess WN's growth means they have now some routes with enough traffic that moving 1,200 passengers a day with four airplanes (and 4 crews) instead of 6 aircraft (and 6 crews) makes sense.
Look at it this way. When WN absorbed AirTran, they dumped AirTrans' 71s for lacking commonality. Delta snapped them up on lease because they fit well with Delta's existing MDs - and Delta was able to replace two 50-seat regional jet flights with one 717. (Their stated reason for taking on the 717s).