Because smoking was banned on airlines in the, US at the end of the 80s...so if it's due to a safety related ban, there would have been a decline in the number of emergency landings due to cigarette smoke being mistaken for an uncontrolled fire.
Also, you seem to assume passengers actually stopped smoking since the ban. Presumably, people smoke less, but at least now it’s more clear to the pilot that something is definitely out of order when there’s smoke.