To a casual fan of Star Wars, this question might not make a lot of sense considering that Darth Maul is a great character and should be featured as the main villain in his own story at some point. But then one has to remember that this is Disney-owned and led now, and not only do they like their leads to be just barely over the line between good and evil, but Maul also isn’t exactly the main villain. Take yourself back to the prequels and remember that Maul answered to Darth Sidious for quite some time, as he was the dark lord’s apprentice and quite honestly his weapon against the Jedi, or one of them at least. Maul was more or less an instrument, a tool that Sidious had honed and lobbed at the Jedi a couple of times before he was cut in half by Obi-Wan Kenobi, proving that he wasn’t infallible and was more or less an impressive character that had a few major failings that could be exploited by skilled opponents that were able to take advantage of his momentary lapses. Plus, Maul wasn’t an entirely stable character since after constructing himself a mechanical lower half he was found to be quite insane until he was taken back to Dathomir where he was made whole, more or less. Even he wasn’t seen to be entirely stable since his hatred of the Jedi was still overreaching and allowed him to make several mistakes as he continued to build crime syndicates and even take on the Mandalorian’s. His power was questioned once again when he and his brother Savage took on Emperor Palpatine during the Clone Wars series. Both of them were soundly defeated, but this wasn’t the end of Maul just yet.
It’s been mentioned that Maul might actually make his way into the Obi-Wan series at some point, where it would be evident once again that he’s more of a supporting character and not a lead. This is a saddening fact to be honest since he’s worth so much more, but the fact is also made by reminding people that Disney wants to do things their way and aren’t really into offering that much fan service when it comes to Star Wars. The Mouse House will do as they will with the franchise since they own it and are going to remind everyone of that fact over and over as much as they need in order to get the point across that Maul and anyone that came along before they took over is now their property, and are to be used at their discretion. This kind of attitude can vilify Disney in a way by showing that they’re not willing to appease the fans, perhaps thinking that they can do something even greater than what Lucas envisioned. But it’s also a good way to alienate a lot of hardcore fans that have been watching Star Wars for longer than the Mouse House has owned it. Some might want to think there’s a power struggle to be had there, but this is kind of silly since it’s not the case in any way, shape, or form. The fans have no real power other than to like or dislike the movies and the shows that might come as a result of Disney’s tampering with the franchise, and even then, there are enough casual fans that happen to like what’s been done to the franchise as of late, and won’t bother taking the time or energy to get to know the EU and everything that’s come before this point. They like what they see, and that’s that.
To Disney, Maul is another piece of the franchise to be moved around where they see fit, and they might not move him around that much, to be honest since his story has been locked in by the prequels and by what has come along already, meaning there’s not a lot of wiggle room to do anything with him at this point. Between The Phantom Menace and the Clone Wars and other appearances that he’s made, there are still plenty of stories that could feature Maul, but making him the main character, not just the main villain, is something that doesn’t feel likely when it comes to Disney, especially considering that they see him as more of a supporting character and, more than that, his story has already been largely told in live and animated form. Taking the time to make sure that his story is told fully and in a manner that doesn’t conflict with the rest of the franchise is a long and tiring process that the Mouse House obviously doesn’t want to go through since it might mean getting creative and even innovative, two things that are a little beyond their scope at times. Oh yeah, I said it.