I think you are probably living in a bubble if you think that those reasons are rare. I'm in Boston (not the most diverse place) and I know a bunch of people in the local industry who don't drink for religious or dietary reasons. Moreover, two of the things you mentioned as being red flags (alcoholism and an allergy in scare quotes) aren't things I would ever consider red flags. If someone is an alcoholic and abstains from alcohol, that's probably a pretty good sign about the person! If someone claims a medical condition which prevents them from drinking, well, I'm not a doctor/health expert so I'm not going to judge that.