Here's why:
* You can't count individual vocabularies: You wouldn't say "I have three vocabularies."
* It's treated as a whole: Vocabulary refers to the entire collection of words and phrases someone knows or a language has.
* No plural form: It doesn't have a distinct plural form like "vocabularies."
Examples:
* Correct: She has a vast vocabulary.
* Incorrect: She has three vocabularies.
Let me know if you have any other questions!