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.