As has been shown in another question, in comparisons with than both the accusative and the nominative are possible and grammatical:
- He loves you more than I.
- He loves you more than me.
However, when we use but only the accusative seems possible:
Nobody will help you but me. [Not I]
What I think is that the nominative case could be justified as well.
Nobody will help you but I [will].
Why I am wrong? Any other examples and references will be appreciated.