Here's why:
* Lay is a regular verb, meaning its past tense is formed by adding -ed to the base form.
* Laid is the past tense of both "lay" (meaning "to put something down") and "lie" (meaning "to recline").
Example:
* "I laid the book on the table." (Meaning "to put down")
* "The dog laid down for a nap." (Meaning "to recline")