I wonder if it happens because the manic symptoms of bipolar are more outwardly evident? Depression is somewhat isolating -- during a depressive episode, a person is more likely to stay at home, withdraw from friends and outside interests. What they are going through is not in full view.

People going through manic episodes are in public view, as it were, often engaging in self-destructive behavior.

One isn't worse than the other, but I think that the privacy of depression might be why bipolar seems to be taken more seriously.