Because all other DC characters are awful.
They've got embaressingly bad costumes, rubbish powers and worse motives, un-iconic and boring villains, poor storylines that only surpass the last by being more ridiculous, and a host of other flaws including, often, flat characterisation; all of which mean that they shouldn't ever get even a film adaption, let alone inclusion within a decent franchise like Batman.
God forbid Nolan's franchise so much as mention them.
Especially Superman, who is completely unrelatable as a character, ridiculously overly powerful, and ridiculously dressed.
Bah.