Gen 2 is a very bulky gen because of how EVs worked in Gen 1 and 2. Instead of having 510 EVs so you could only max out two of your six stats, they were called DVs instead and could max out all six of your stats as well.
Crit was no longer based on speed and had a set standard percentage.
They nerfed sleep, which also buffed rest. Then they introduced Curse, which basically everyone could learn and was really good due to how DVs worked.
Then with the introduction of held items, and how DVs wroked, Leftovers as basically the best item to have in most situations.
Snorlax got buffed in a way do to the special split, gaining high special defense and use was a perfect candidate for curse. Because of its combination of stats, and Dark/Steel not nerfing Psychics/Normals as they intended it . Snorlax could also carry Fire Blast in case Skarmory/Steelix tried to wall it. It was a good time to be a Snorlax as the only Pokemon that could reliably OHKO it was Machamp.
tl;dr: Snorlax is the best Pokemon of GSC OU and it honestly should have been banned as almost everything in OU is OU on how it revolves around Snorlax, whether its reliably walling it, reliably killing it, reliblying walling/killing the things that threaten Snorlax or making Snorlax's job easier in the long run.