Answered by Theodore Kornweibel:
Garvey's downfall was the result of (1) persistent efforts of the U. S. government to prosecute him, with the ultimate goal of deportation; (2) Garvey's and the UNIA's own mistakes and mismanagement; and (3) strong opposition from important segments of the African American community.
The government was determined to find any means by which to prosecute him (including attempts to pin Mann Act charges on him, during the time when he was having an affair with Amy Jacques while still married to Amy Ashwood). The chaotic finances of the Black Star Line (BSL) and other UNIA enterprises, including advertising passage on a ship not yet owned by the BSL, provided government prosecutors with a gold mine of damaging information.
When Garvey acted as his own attorney during his trial, he damaged his case. These events did not take place in a vacuum, however: Garvey was the object of persistent and often valid criticism from much of the black press and from the black socialist and communist movements, which were particularly outraged by his meeting with the Ku Klux Klan and the loss of hundreds of thousands of dollars in contributions and BSL stock purchases.
Finally, Garvey was the charismatic "glue" which held the UNIA together. Once he was imprisoned and then deported, he was unable to rally and inspire his American followers--the backbone of the UNIA--and the movement went into decline.