Referee selection : Why would an editor pass up inviting the most suitable referees?
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.