I think the explanation is that Lily used old magic that nobody else knew about. Like, it wasn't just her love, it was a special spell powered by love, in the same way the killing curse is powered by hate. Like, imagine the government is coming to bulldoze your house to build a freeway bypass, and you're such a big history nerd you pull out some legal precedent from the time of Arthur that says they can't do it. It was a highly situational spell that nobody bothered to teach to the next generation because you can only use it once, and she just happened to read the most history books on obscure spells.
Alright I'd better insult this series to make up for defending it. Putting Santa hats and beards on the severed slave heads is ghastly.
Which means Harry is protected by the power of ritual human sacrifice, just like Voldy.
