The latest thinking is that alcoholism is a combination of genetic and environmental factors. It seems to run in families though it may skip a generation or two in the right environment.
I had a pretty good upbringing, no abuse or anything like that, and my mothers alcoholism wasn't really apparent to me until I got sober.
I gave up trying to figure out the why quite early. It seemed to me even if I knew why, it wouldn't help me much and would probably just give me someone to blame, which would be even less helpful.
Instead I concentrated on the solution I found in AA, and haven't needed to drink since, no matter what.