1. He will never rule something as large as he did in Clone War

Maul was a direct ruler of a star system, Mandalore. Also with Hutt Cartel, Pyke Syndicate, Black Sun joining his "Shadow Collective". He possessed direct bureaucratic, military and established legal control to every part of the society including government, citizens and law in the star system. He even had Hutt Family summited to his "Shadow Collective" authority, the family which directly controls more than one star system and its influence to galaxy so large that even Empire doesnt want to mess with. The political power Maul possessed at that time is so big that not only Jedi Council view him as absolute priority threat, but also his master Sidious to view Maul as a rival and had to risk exposing his own identity for eliminating the threat.

Now Maul cannot directly influent a star system anymore, and Hutt Family left the alliance, Maul can only operate in shadow solely, he still created impact to the galaxy but not as large as he did in clone wars, and not even larger than the impact Qira posed afterwards, which even Darth Vader commented: "Qira is certainly brighter than the dim light of Maul" in Hidden Empire #4. 

2. He will never had any truly loyal follower/allies who stood with him no matter the circumstances anymore

During clone war times, Maul has powerful people that are extremely loyal to him. Mother Talzin, Savage Oppress two powerful force users at his side. Gar Saxon, Rook Kast, two of the high reputated clans and noble families in Mandalore, backing Maul at all cost even after Maul was utterly defeated by Sidious and imprisoned in Separatist Prison Spire. Those people are the strongest allies and followers Maul can get. Now Maul finally has his new apprentice at his side, Devon, who we knew will backstab Maul at some point when Lawson reunited with his son and told her the truth. Maul will only get more lonelier and more vulnerable as he progressed further to his life when literally everyone he encountered will backstab him at any moment. And once he is injured, we know the rest, he can't even deal with the inquisitors.

3. Maul will not have any great Mandalore warriors fighting for him

Mandalore warriors are considered the most elite, feared, and respected fighters in the Galaxy. Yes Maul still has the darksaber which represent the ancient, revered symbol of Mandalorian leadership and unity. But it is basically end of ties between Maul and the Mandaloreans, after Rook Kast, the leader of Clan Rook and House Kast, who is the fiercest, the most loyal, and being last remaining Mauldalorian follower of Maul, dead. There will be no more high tier elite Mandalore warriors joining him, but only the lowest level of scattered mandalorian mercenaries who swore no allegiance and no honor as a warrior. His darksaber becomes nothing but merely an empty symbol until Ezra picked it up and restores its meaning.

4. Maul will never get the level of revenge like he did in Clone Wars

Maul's goal of his life is all about getting revenge. And his target is always the two, Obi Wan Kenobi and Darth Sidious. In Clone Wars, Maul actually hurt Obi Wan himself, torturing him by killing the one he loves, letting him suffer forever in his life. He inflicts emotional wound to Obiwan that will never heal. It is like one of his life goals completed. In Shadow Lords, maybe he will injure Darth Vader with the help of Devon, but to what extent, and what about sidious? Even Darth Vader commented: "Qira is certainly brighter than the dim light of Maul" in Hidden Empire #4. Yes Maul set up a foundation for Qira to develop further and ultimately hurt Sidious by aiding the rebels.

Maul certainly completed his life goals after his death. But during his life, his best moment is definitely THE CLONE WAR, considering the pain he caused to Obi Wan and the fear he has given to his master Sidious. With the 3 main reasons above, Maul couldn't be more vulnerable, less resourceful, less reputed but just more miserable as he progresses his life.