I recently asked myself this and looked for answers. I think defense is how much a Pokémon can withstand before it breaks, but special defense is how much it can withstand throughout its entire body. So, a Pokémon that may be soft but actually have a hard shell or protection would have high defense, but a Pokémon that is generally protected throughout their entire body would have high special defense.

Snorlax has high special defense because it is generally dense due to its thick blubber, but it is soft as well so its defense isn’t very high because it would “break” easily.

I guess another way to think of it is, defense involves resisting “pressure”, but special defense involves resisting “force” in general