The cable net comes in through the corners and goes right through the middle of where the terracing would be. It would be obstructive and dangerous to have a random cable coming in which would basically be in reach of anyone sat there.
The tension is needed at the corners and with expansion behind each goal, can’t really be supported else where in a way which would be even considered financially viable for such a small capacity increase.
The best we’ll see is them adding in more seats using the space currently available which they’ve already done to a degree at the South Stand end.
Ultimately, it’s why non of the 4 corners have been done. If the corner cable net entries weren’t a problem then the new North Stand would likely cover the comers as well since the working design uses the absolute most space possible for the highest capacity.