Here's why:
* It refers to a single concept: Peace is a state of tranquility and harmony, not a collection of individual things.
* It doesn't have a plural form: While you might say "peaceful times," "peace" itself doesn't have a plural.
* It functions grammatically as singular: You would use singular verbs with "peace": "Peace is essential for a healthy society."
Even though it represents a desirable state that applies to many people, it's still a singular concept.