It's because GenZ is addicted to nicotine and millennials are all using legal weed. We've just replaced one substance with some others, rather than started abstaining.
Which IMO speaks into failure of tobacco control and why "tobacco harm reduction" is a lie (dogwhistle, even) designed to sell and even invent more neo-nicotine products (vape and pouches, among others) while distorting smoking statistics (and statements - people saying they "quit" by switching to neo-nicotine products, while in reality they just switch their addiction intake).