I'm wondering why are mean drunks diagnosed bipolar?

Not automatically: People's behavior when intoxicated is highly variable -- and "mean drunks" are not automatically diagnosed bipolar. If someone has bipolar disorder though, chances of alcohol further destabilizing mood are high. I've also seen many alcoholics who are "mean" or irritable when intoxicated, but who are not bipolar.

Not necessarily so: Having an alcohol problem does not mean that a person is bipolar. However, some individuals who are bipolar self -medicate with alcohol.