zoros parents must have abandoned him atau died because in his flashback he was wandering from place to place, sanji's dad might be Zeff? they never quite cleared that up, Nami's parents died in the war and she was rescued blah blah blah u know the rest, Franky's mom might have been the train lady? just a guess again, also brooks parent weren't mentioned, just an afterthought
I was wondering that too, I actually wrote an artikel about my theories on who their parents were. I think that's something that Oda may reveal later. However, it's not very crucial to the story right now. All we need to know for now is just the people who raised them i.e. Sanji: Zeff, the people at Baratie. Zoro: Kuina, Koshiro. Franky: Tom, Iceberg, Kokoro. Nami: Belemere, Nojiko.
Franky's dad abandoned him and that's why Tom-san let him live with them (Iceberg, Baa-san and Yokozuna). It was also why Tom never told him off for building weapons. Nami's parents died, I believe, so Bellemere adopted her. Zoro and Sanji's parents aren't mentioned, so I assume that Oda wishes the audience to consider the people they grew up with as their foster parents, so to speak. It could be that Sanji and Zoro's parents died.
We can answer two of those questions. Frank's parents are mention in his flashback. they abandoned him because Franky was annoying and they were asshole pirates. Tom is a better parent anyway. Nami parents were massacred sejak pirates when she was found sejak Bellmere. As for Zoro and Sanji it still remains a mystery, though Zoro may be an orphan too as we see in his flashback(anime) he looks poor. Sanji is lebih complicated since he seen as a kid was on a passenger ship Orbit BUT says he was born North Blue it's got somthing to do with his WANTED ALIVE? poster....
I think Franky was an orphan and tom the guy who made emas D rogers ship adopted him. Nami's parents where killed in a battle between pirates and the navy and bell-mere took pity on her and nojiko and dissected to adopt them