Here's why:
* Incomplete thought: The sentence is missing the object of the verb "treated." We need to know what they treated us to.
* Missing article: You need an article (a, an, the) before "great" to make the sentence grammatically correct.
Here are some ways to correct the sentence:
* Treated us to a great meal.
* Treated us to a great time.
* Treated us to a great performance.