If you are able to watch the interviews after the documentary, they discuss why they are just now talking about it and previously defended Jackson. It is important to note that the documentary director approached them and they were not paid for participating in the film.
This article from Vanity Fair was insightful too. These two men are not the only boys to have accused Jackson of this - there are at least 3 others who have come forward. Who knows how many others are out there but silent?