Assuming the official account is accurate, we have no better explanation for what happened than Rowling's love shield(though I've heard the closely related theory that it was Voldemort breaking his promise to Lily that did it, because the laws of magic somehow enforce contracts). MoR!Voldemort is not the sort to leave it as an enigma, so he's likely gone looking through obscure magical texts of the sort that he didn't check pre-death to figure out what had the power to do it. As such, he would presumably have learned the importance of true love and/or honesty, and altered his tactics accordingly, which may be why Quirrelmort is noticeably less evil-acting than Voldemort.