Here's why:
* Subordinating conjunctions connect a dependent clause to an independent clause. They introduce a clause that cannot stand alone as a sentence. Examples include "because," "although," "since," "while," etc.
* "In addition" is an adverbial phrase that indicates something extra is being added to what was already mentioned. It doesn't connect clauses or create a dependent relationship.
Example:
* Incorrect: In addition, the weather is sunny. (This doesn't create a complete sentence)
* Correct: I went to the park, and in addition, I bought some ice cream.
"In addition" is used to introduce extra information, but it doesn't create a subordinate clause.