Here's why:
* Transitive verbs require a direct object. A direct object is the person or thing that receives the action of the verb. For example:
* "She wrote a letter." (The letter is the direct object)
* "He kicked the ball." (The ball is the direct object)
* "Is" is a form of the verb "to be", which is a linking verb. Linking verbs connect a subject to a noun or adjective that renames or describes it. For example:
* "She is a doctor." ("doctor" renames "she")
* "The sky is blue." ("blue" describes "sky")
Since "is" does not require a direct object, it is not a transitive verb.