Because he thought that it is better to ensure that people understood the main reasons why Martin and the other parts of the movement did what they did.
Explanation:
In writing the letter, King tried his best possible to explain the main points why he drafted the letter. He wanted to share the vital information about Martin and the movements by others with the public to ensure that they understood the main reasons why they performed the specific actions. Thus, he put the main points in the beginning of the letter.
