Here's why:
* Coordinating conjunctions connect words, phrases, or clauses that are of equal grammatical rank. They join elements that are grammatically similar. Common coordinating conjunctions are: for, and, nor, but, or, yet, so (often remembered with the acronym FANBOYS).
* "Until" is a subordinating conjunction. It introduces a dependent clause (a clause that cannot stand alone as a sentence) and shows the relationship between that clause and an independent clause.
Example:
* Coordinating conjunction: "I went to the store, and I bought some milk." (Both "went to the store" and "bought some milk" are independent clauses)
* Subordinating conjunction: "I will wait until you arrive." ("Until you arrive" is a dependent clause, and it depends on the independent clause "I will wait" for its meaning.)