Why does predecessor(zero) need to be zero in Church numerals?
The other reason (than the one already stated) for pred 0 = 0 are the (all too often unstated) requirements that


pred be a total function from nat to nat
pred succ n = n be true.
pred be as close to injective as possible.
pred be as close to a monotone decreasing function as possible.


It is that last requirement which really 'seals the deal' in forcing pred 0 = 0, since that is the choice which minimizes the distance to not being monotone decreasing.