The reason was partly explained in TCW and then explored further in a novel:
Really though when I first saw Maul holed up on a junk planet with spider legs as such, I didn't question it, because it was fantasy, I went with the flow. And it fit with the fantasy feel, this idea of demons coming back to haunt you and such with mysterious explanations. The Force doesn't make sense either, then they explained it...
I realise this is a matter of perspective, but we can't forget this is a fantasy universe where the impossible is possible and doesn't always need an explanation, which in part IMO enriches the fantasy experience.
P.S. The Maul related episodes, especially The Lawless & Revenge are widely regarded as some of the best episodes of bunch, and Maul is definitely one of my favourite TCW characters, but each to their own I guess.
P.P.S. I do hope you didn't avoid those episodes just because of this, because that would be silly.
Even in fantasy and in sci-fi they have established rules in the universe, things people can and can not do. It was out side the rules the univer set up.
And of course I did not avoid any of those episodes. When I heard how well regarded they were by the fan base I had to see if the execution was what every one talked about and if it was successful in doing what i feared it wasnt.... and for me it just wasnt. They barely glance over him being alive and the best moments for me were from the characters the show introduced NOT MAUL. I liked the criminal (drawing blank on names right now) and the loss of the Mando queen was very emotional, but every scene with maul for me was me waiting for him to grow a mustache to twirl to the point I think me and my brother actually started riffing on it. We watched it multiple times simply cus we were to busy laughing the first time.
Edit: Hando... still dont remember the name of the queen.