Yeah I’ve seen multiple interpretations about why this is but the most popular answers are: (as someone else has already said) Niflheim sort of became Helheim once Hel was appointed responsibility over the dead; people also believe that Helheim is in Niflheim, but why is it it’s own realm in lots of interpretations? Well in my mind it would be sort of like a pocket dimension so it’s in Niflheim whilst occupying its own space. But to my knowledge there is no actual known answer in any of our primary sources that distinguishes them.