Why do some Penguin Classics have footnotes but others do not?
Can you give us examples of what you mean?
Because the editor felt footnotes were needed for some but not others.
I am not sure what you are asking – Penguin Classics have been published since Og was painting on the walls of his cave. Different editors, different publication years, even stylistic concerns mean different decisions on what needs further explication.
There’s no good single answer. Some classics really require footnotes – The Exeter Book Riddles and The Stories of Edgar Allen Poe (especially things like The Thousand and first Tale of Scheherazade). Others you would think would profit from copious footnotes and and have very few. There tend to be a lot in the translations of classical works.
For some reason some, like Moby Dick, have admirable and copious notes, while others, like War and Peace, have much fewer. There seems to be no rhyme or reason to much of it.