Hi there !

I have seen written in a few articles "zira". I know it means "because" so the same as "çünkü".

I have the feeling "çünkü" is really used more often.

Is "zira" is used in more formal context ? Or is there no difference at all ?

Hello!

Yes, çünkü is used more often. "Zira" is an old fashioned word in Turkish. That´s why it is not used much. It is used in formal texts sometimes, but since it has same meaning as çünkü, we prefer using çünkü.