Why didn't the monster eat the crazy person?

“Because the crazy person was too fast”

Monsters are typically depicted as powerful and not easily outpaced, so speed wouldn't usually be a logical reason for a monster to avoid eating someone.

“Because the crazy person scared the monster”

While the crazy person might be unpredictable, monsters are usually portrayed as being fearless or more intimidating than their prey.

“Because the monster was on a diet”

Dieting doesn't typically relate to the behavior of fictional monsters in riddles, where the punchline usually plays on a humorous twist rather than a concern for health.