Weight, meaning if they had to replace the LiPo batteries with a NiCad, they would be twice as large and 4 times the weight.
Now factor in that there are little other batteries all over the plane as backups for stuff like...flight controls since there are no hydraulics. Yes there are the main and APU batteries, but there are more, all over. Electric jet...
You quickly start eating into payload that is already on the thin marigin of what Boeing specified for fuel efficiency. Now you have to start paying contract penalties for non-performance. That gets expensive. Hence, my comment of weight prohibitive.
So which will weigh more? A steel box for all the batteries, or a completely redesigned system with NiCads/new battery chargers? That is the design excercise Boeing is going through righ now, and how long other options would take, and get the FAA to buy off on it.
And I'm sure there is more I don't know about. Large design gamble with untested technology--bit them big time. Boeing has to build a new battery system, and we have the conservative FAA. Airbus just tells their customers, bring your plane on back before 150 hrs so we can rebuild the wings before they fall off. Different agency, different sources of income/payoffs.