There are so many reasons why Darth Maul could have survived you couldn't spit without hitting one.
1. It's Star WARS. We're talking about a STRANGE universe here with droids that have more personality than most PEOPLE, human beings can understand dozens of alien languages some of which are pronounced with vocalisms that aren't even possible with human anatomy, and an entire society of people wielding strange powers that allow them to do extraordinary things like shoot lightning from their arse and choke people halfway across the galaxy in open displays of raw power.
2. Darth Maul was not a human. He was humanOID in appearance, but technically, we know diddly squat about his people other than the fact that if any race of beings could be considered "evil" based solely on appearance, the men of the planet Dathomir certainly are that.. hell their planet's name actually has DARTH in it. For all we know, his people may be like sponges.. cut one part off they keep on living. Who knows what their anatomy is like.
3. The dark side of the force has odd powers, even so much as powers over life and death. We've seen the ability to reanimate the dead by the larvae of the queen of the Geonosians. We saw Anakin burned literally alive in lava and survive. Perhaps the hatred that fuels the Dark side of the force doesn't allow those who worship the dark side of the force to go peacefully into death, so they cling on with their hatred making them resilient.
4. Lightsabers are hot. They can cauterize a wound the second they cut through flesh, so.. well that's something.
5. The major rule when fighting evil... ALWAYS check the body and make sure they are dead. Far too many "would be heroes" have had their heroic careers cut tragically short by forgetting this single major lesson.