While there were hopes for an appearance in Obi-Wan Kenobi, Lucasfilm's decision to cut Darth Maul's return after the series was reportedly reworked was a smart move, preventing a sizable retcon that could have occurred. Star Wars fans know Maul survived his first duel with Kenobi during Star Wars: Episode I - The Phantom Menace and went on to become his nemesis during The Clone Wars. However, they've also had their last duel as seen in Star Wars Rebels, leading to some challenging canon hurdles Lucasfilm would've had to traverse if had returned in Obi-Wan Kenobi.
Following his first defeat at the hands of Kenobi on Naboo, Maul was eventually found by his brother during The Clone Wars, after which his mind was restored and he was given new legs. Having been replaced by Darth Sidious with Count Dooku, Maul and Savage determined to seek revenge on Kenobi throughout the Clone Wars while also building the criminal enterprise that would eventually become Crimson Dawn following the rise of the Empire. Having failed to kill Kenobi during the Clone Wars, Maul feared Kenobi had fallen during Star Wars: Episode III - Revenge of the Sith with Order 66 and the Jedi Purge.
Following his leadership of Crimson Dawn from the shadows, Maul eventually came into contact with Jedi survivors Ahsoka Tano, Kanan Jarrus, and padawan Ezra Bridger in Star Wars: Rebels. Through his manipulations of Ezra, Maul was able to learn not only that Kenobi was alive but also his location on Tatooine. Once having finally found Kenobi at the end of Rebels' third season episode "Twin Suns," Maul and Kenobi had their final duel in the Star Wars timeline, taking place only a few years before the events of New Hope, where Maul finally met his ultimate demise. As a result, bringing Maul back for Obi-Wan Kenobi ten years after Revenge of the Sith would have presented a significant continuity challenge, seeing as how Maul didn't know Kenobi was alive or knew where he was until years later during the events of Rebels.
While a scenario where Obi-Wan leaves Tatooine to fight Maul could have happened (where Maul then believes Kenobi to be dead once more prior to Star Wars Rebels), reports have suggested that Obi-Wan Kenobi underwent rewrites because it was too close to The Mandalorian with a warrior protecting a child. This seemingly implies that early drafts had Kenobi staying on Tatooine to protect Luke, perhaps from Maul. It's a story that certainly sounds like a significant retcon of the events in Star Wars Rebels.
At any rate, Obi-Wan Kenobi's pivot to Darth Vader as the series' big bad allows for some greater and exciting freedom. Based upon the first tailer, Obi-Wan will seemingly have no choice but to leave Tatooine to keep Luke safe from the Imperial Inquisitors, exposing the truth of his survival to the Empire before finding a way to stay hidden once more to protect the galaxy's future hope. Obi-Wan's story with Darth Maul has already been revealed and told (with one of the greatest lightsaber duels ever). As such, Obi-Wan Kenobi putting its focus on a rematch between Kenobi and his fallen apprentice feels like the stronger route where new continuity can be created (rather than attempting to weave around or rewrite it all together with Maul).
