Why did so many African American leaders reject Marcus Garvey?

Simply put, Marcus Garvey was seen as being too radical. Most political leaders within the African American community were committed to a strategy of cooperation with whites. But to Garvey, any form of compromise with the dominant race was anathema. He advocated a complete separation of the races. If African Americans could not physically leave the United States, they could at least establish a separate cultural identity, one built upon a sense of radical difference with white America.
In large sections of the African American community, there was also widespread mistrust toward the almost messianic fervor with which Garvey articulated his radical message. Mainstream African American activists were focused on more practical considerations, such as dismantling the Jim Crow laws. To them, Garvey's political ideology was wholly unrealistic, unable to deliver tangible benefits to African Americans desperate for legal and political equality.