Here's why:
* Direct objects receive the action of the verb. They answer the question "What?" or "Whom?" after the verb.
* "Kind and helpful" is a description, not a recipient of action. It's an adjective phrase modifying the subject of the sentence.
Example:
* "He is kind and helpful."
* "He" is the subject.
* "Is" is the verb.
* "Kind and helpful" describes the subject ("he").
Let's look at an example with a direct object:
* "She gave him a kind and helpful smile."
* "She" is the subject.
* "Gave" is the verb.
* "Smile" is the direct object (what she gave).
* "Kind and helpful" describes the smile.