As IQV already said: There is no etymology behind that. I think that the reason for this is the colloquial language. If I talk about Munich (München) and people from Munich (Münchener) I won’t pronounce every syllable. If I say Münchener it sounds like Münchner.
Maybe its comparable to wanna instead of want to in English.
I don’t know the exact reason of the naming of this newspaper — but I found this little Q and A. The question is exactly yours and the answer comes from a person who deals with Schwizerdütsch (Idiotikon). He says, that Züricher and Zürcher were equal in literature over centuries and Zürcher has something to do with pronounciation.
One more thing: I found a forum where a person from Zürich explains, that people from this city always call themselves Zürcher — and that this little difference is quite important to them (3rd post).