It’s true that more people die from alcohol and tobacco. But that doesn’t mean that marijuana is less dangerous.
Marijuana causes major mental health problems, from anxiety to psychosis (ever heard of druggies who are “burn outs”–who have basically lost their minds? It happens). There is also growing evidence of its addictiveness.
It negatively affects short-term memory, dulls your mind, reduces motivation, and makes you focus on physical pleasures, while sometimes giving you the false impression you’ve come to some great insight, which turns out not to make any sense when you try to tell people who aren’t stoned—all bad things for one’s avodas Hashem.
Drunkeness is very bad, and totally assur (as is all tobacco use), but small amounts of alcohol have safe and predictable effects.
TLIK: You’re right there’s no real justification for getting drunk on Simchas Torah, and even on Purim getting drunk is prohibited according to many authorities.