Referee selection : Why would an editor pass up inviting the most suitable referees? This question is related to this one, and is dual to it in some sense. 

Assume PIs A and B publish a few key papers in what turns out to be a new mini-area. Their work is well received, and they arrive at similar conclusions using complementary techniques.

Several months later, A notices an in-press paper by C, which claims to significantly improve upon the work of A and B. The paper by C is substandard, however, reflecting an evident lack of understanding. 

On alerting B, A discovers that C's paper was not reviewed by either of them. 
A and B agree that C's paper makes a strong, unsupported claim, and that they would certainly have flagged it as problematic (as reviewers).

What reasons could a handling editor have for choosing neither A nor B as referees, given the high relevance of their work? Replies from people with editorial experience would be especially helpful.
Many possible reasons, although there is of course no way of finding out which one applied here (if any at all):


Editor E believes that, being big animals in the field, neither A nor B will have time to review.
Variation: E is a junior researcher and doesn't think that he has the stature to ask A or B for a review.
E is not on good terms with A and B.
E believes that A and B have some sort of personal problem with the author and doesn't want to put them into a position of a conflict of interest.
E has recently given both A and B difficult papers to review and doesn't want to overload them.
The author has asked for neither A nor B to be reviewers.
E simply has not paid attention to the previous seminal paper and so asked colleagues he knows are well versed in the general field.
E actually knows the field well herself, and so self-assigned herself the paper, plus a couple of grad students who should learn how to review papers.