You should be a bit more skeptic, when we lost our light in the red war no guardian regained their memories, plus Eris was speaking strangely. I bet that was curia.

Think about what happened to Ana Bray. She left her people and the City to go dig around in CB facilities for years, and wasn’t serving the Vanguard or, if you want to get particularly technical, the Traveler by defending it. It could also be to keep us from remembering the Collapse or how we died. Think about it, the civilians are the only ones who could remember what happened during it. Imagine if they had to fight these creatures that killed billions in their lifetimes or just before. The only fear Guardians feel is from their own experience on the battlefield, if they had to tote around memory of the deaths of friends, family, or maybe themselves, I doubt many of them would be able to fight. Although I do have to say, giving Guardians memories is still a bold move. Imagine if one of them remembered something really tragic, or a hidden truth about someone like the Speaker and revolted about it. If Osiris could gain a cult following from a theory like that, imagine what a memory would do...

Ok, Ana Bray

My guess is it doesn't want guardians to be held back by their previous life, or the light has a price (Ghaul being an exception because of how he took it)