Here's why:
* Coordinating conjunctions connect words, phrases, or clauses of equal grammatical rank.
* "But" shows a contrast or opposition between the elements it connects.
Example:
* "I wanted to go to the beach, but it was raining." (The phrase "I wanted to go to the beach" is contrasted with the phrase "it was raining.")
Other coordinating conjunctions include:
* and
* or
* nor
* for
* yet
* so