I think the big gateway drugs are alcohol and nicotine, I don't know many people who have been/are on hard drugs but they all started on nicotine & alcohol.
The only common trend I've personally noticed is that smoking pot tends to lead to nicotine addiction at some point, but this may be a UK thing.
That may be emblematic of the no-smoking era. With less and less people smoking you weed (pardon the pun) out the soft-core users. If you looked at the percentage of people who smoked cigarettes in the 1940s-1960s who became drug users of any kind you would find it a miniscule percentage. I would suspect it would be similar with alcohol, but not as small a group, and that was a period of hard drinking and heavy smoking.
I don't think its alcohol or cigarettes that does it. I think that there is a small percentage of people who are predisposed to addiction, and another group that find themselves there because of several factors, not the least of which is societal acceptance.