Here's why:
* "Language Arts" is a proper noun, specifically a compound noun referring to a specific subject area.
* "language arts" is the common noun form, referring to the general concept of skills and knowledge related to language.
Think of it like this:
* "English" (proper noun) vs. "language" (common noun)
* "History" (proper noun) vs. "history" (common noun)
Just like "English" is a specific language, "Language Arts" is a specific subject area. But when we talk about the broader concept of language skills, we use the common noun form: "language arts."