The necessary condition X of event A means, given event A, then X necessarily occurred. If cause is a necessary condition, then it is follows that all causes are necessary to its effects.

But that's not the case, since it is plausible that an effect A could be caused by a number of different causes. None of them could then be said to have occurred based of the observation of the effect. So, in my opinion, it is more sensible to relate cause with what provides sufficient conditions, not with necessary conditions.