I remember hearing as far back as 2007--long before Walking Dead crowds--that IoA simply didn't have the capacity for the event, especially with all of Seuss Landing shuttered. Universal is the bigger park, why I think it will always by necessity be a part of HHN.
I'd just add, having walked through an HHNized Seuss Landing two different years, it was kind of creepy and novel the first time but quickly became "bad show." Too much dead space, no pun intended. Literally nothing open. No scare zone, no rides, no bars, no snack carts.
(...)
If you have to use the Studios for capacity as well, the crossover becomes the main issue. It's a long-ass walk, requires security, extra ticket takers, maybe some scare actors... I think that would be the only real issue.