Here's why:
* Coordinating conjunctions connect words, phrases, or clauses that are of equal grammatical rank.
* Subordinating conjunctions connect a dependent clause to an independent clause.
Example:
* Coordinating: "I like pizza, but I don't like mushrooms." (Both "I like pizza" and "I don't like mushrooms" are independent clauses.)
* Subordinating: "I like pizza because it's delicious." ("Because it's delicious" is a dependent clause that relies on the independent clause "I like pizza.")
In the sentence "I like pizza, but I don't like mushrooms," "but" joins two independent clauses, making it a coordinating conjunction.