Why cigarettes are common all over the world?

Okay, well if this is a general question, I believe the popularity of cigarettes is mainly due to the over-commercialization and distribution of them. People have smoked tobacco since the dawn of time, so it is not a new thing. However, whenever companies began producing specialty cigarettes, you only saw the elites with them. This is due to the fact they were "new" and therefore "cool' because only the higher classes had access to them.

As time passed, and cigarettes became mass produced, people quickly flocked to purchase them so they could appear to be a part of the "higher social class", as you may know, cigarettes contain nicotine, a highly addictive drug. Once people started smoking for social distinction, they quickly became addicted and started smoking for a "fix".

Ever since then, smoking has gained popularity across the world do to its effects. Basically meaning cigarettes are a legal, marketable drug, that is easy to get a hold of. 

Hope this helps!