Didn't you forget another twig on the branching rejection tree? That is, isn't it true that most editors don't have carte blanche to publish the manuscripts they discover and adore in the slush pile? -- they have to pass the manuscript by a committee of editors, and a marketing team, and an accountant, and the janitor, who may all say "We don't like this," or "This won't sell." Well, maybe not the janitor.
Or would you have contacted the author with positive feedback by this point, so that it wouldn't count as a mysterious rejection?
This is correct. Until we've got our own imprint or are running the department, we have to get positive feedback from the team. It doesn't have to be universal, of course. One of the other editors having doubts about it is different from Sales or Marketing having doubts about it. But if I like something enough to show it to the team, and the team doesn't get behind it the way I hoped, I will send a very nice letter to the author.