Why do reviewers need to be selected again for a revised paper?
I think that two explanations are possible:


(At least on of) the reviewers did not agree to review the paper again. This can happen, as a reviewer you are not obligated to review a revision.
The status on the webpage does not reflect the actual status the paper is in. E.g., the actual status might be 'paper lying on the editor's desk, collecting dust'.