Why did Malcom X leave the Black Muslim movement? What is his reason?

There are many reasons why he, Malcolm chose to leave the Black Muslims. (Biography - An abridged biography of Malcolm X)
One of them was because of a pilgrimage he made to Mecca, which proved to be life altering for him. After he returned to the US he resigned his position in the Nation of Islam. His relationship with the nation's leader Elijah Muhammad was strained, and some say he was marked.

The Bible says in (Jeremiah 10:23) "I well know, O Jehovah, that man’s way does not belong to him. It does not belong to man who is walking even to direct his steps".

Malcolm, who was a firm believer in what he thought was truth at the time, found it hard to tolerate belief in something he found to be untrue.

The Bible also say's, "Do not put YOUR trust in nobles,
Nor in the sons of earthling man, to whom no salvation belongs" Palms 146:3

There are many people who would prefer to follow, the creation, (man) and believe him, rather then listen to the Creator (God, Jehovah, Palms 83:18) and follow what he tells us to do at John 17:3 "This means everlasting life, their coming to know you, the only true God, and the one whom you sent, Jesus Christ".

Only God can read hearts, and I am sure he knew what was in Malcolm's.

On the subject of, "What does the Bible teach", please see: www.jw.org