Here's why:
* Intransitive verbs do not take a direct object. They describe actions or states that do not directly affect anything else.
* Transitive verbs require a direct object. They describe actions that directly affect something else.
In the phrase "was crying," the action of crying is happening to the subject (the person who is crying) but doesn't directly affect anything else. There is no object being acted upon.
For example, "The child was crying" is a complete sentence with an intransitive verb.
Let me know if you have any other questions!