Maul is just weak mentally/emotionally. Physically his only equal is Anakin, his potential in the Force was enough that he could surpass Sidious. Even after being cut in half he is very powerful, enough to rival a prodigy like Dooku even though Dooku had far more training and experience.
But post-Naboo he is just... not all there. Being rendered obsolete and then betrayed by Sidious broke him to the core. That's why everything he touches turns to shit.
Notice that when he is focused he is still very competent, and you can see flashes of his former brilliance. His casual smackdown of Savage, him defeating Kenobi, his fights and schemes in Son of Dathomir. Even in this episode, in their first fight in the tunnels he casually beat Ashoka and could have easily killed her while she was stunned.
But he always loses focus. If he starts getting his way then he quickly grows overconfident and gets humiliated. If things start going against him, it's even worse...he dissolves completely.
That's the real reason Vader is stronger than Maul. Vader can deal with his despair and self-loathing better. Maul's death wish is plain to see at the end of the episode. Of course you can't get anything done if subconsciously you just want someone to put you out of your misery.