Here's why:
* Transitive verbs require a direct object. The direct object is the thing that receives the action of the verb.
* In this sentence, "cafeteria" is the direct object. It's what "you" ate.
However, the sentence is grammatically incorrect. The word "cafeteria" should be replaced with the food eaten in the cafeteria.
Here's a corrected version:
* You ate lunch in the cafeteria.