The process of adding and removing sites is based upon the content of the site in question.
No editors looks at sites A, B & C and then decides to only list B & C because they are 'better' than A.
A site is added because, in the opinion of the editor, it offers unique content and its inclusion will add value for the people who use the directory.
Sites are removed if, in the opinion of the editor, they no longer meet the above criteria.
(Switching to the thinking out loud mode)
Every site owner (not just you) who has a site that no longer appears, really needs to ask themselves the tough question: does my site really stink?
And let's say, hypothetically, that site A really stinks and gets delisted. How does an editor convey that? Imagine the uproar if they told the truth: the site in question is a putrid mass of spam and it was delisted to put the electrons out of their misery. Not your, of course, just hypothetical site A.