Only Amazon knows. When I first started, there were all kinds of theories about it. But, over time, it became clear that some got it with very few accepted hits - others with millions
and one even got it after their account was suspended (couldn't use it, but had masters.......).
We came to believe that it's a lottery and it's random. You probably have to have a certain number of HITS approved - but who knows. FWIW, it does seem as though losing masters is NOT random and is influenced by rejected HITS and/or blocks by requesters.