Here's why:
* "But" is a coordinating conjunction. It connects two grammatically equal parts of a sentence.
* Coordinating conjunctions are not capitalized. They are considered part of the same sentence structure and are not given independent emphasis.
Example:
* Correct: I like pizza, but I prefer pasta.
* Incorrect: I like pizza, But I prefer pasta.
Exceptions:
There are a few rare instances where you might capitalize "but" after a comma:
* If it begins a new sentence. This is uncommon, as "but" typically signals a continuation of the previous thought.
* In extremely formal writing styles. Some extremely formal writing styles might use capitalization for emphasis, but this is not standard practice.
In general, remember that "but" is lowercase after a comma.