Hard to say Michsm, there are definitely "binge" style drinkers that don't drink every day but have cycles like your ex-husband did. It's also fairly normal for drinking to get progressively worse and creep into the week/workday/etc. It's also possible that he was drinking during the week and you just didn't know - alcoholics are masters of hiding/keeping secrets.
At any rate, the answer to "why" we are alcoholics, whether we drink every day or just on weekends - is not really known. I personally had to simply accept that I could not drink any alcohol at any time in order to get better.