Chapter 477 War Engine
Other than The Mechanicum, there were other major underworld organization. An organization would only considered major if they could at least control one Dark City. These organization were Anarchy, Ash Corps, Virus Undetected, and Automatic Sinner.
Each of them controlled one or more dark city and could ask for 'tax' however they wanted in these Dark Cities. The catch was they had to defend it and protect it if they wanted to make profit. If people were getting robbed and attacked constantly, people would stay away from their cities.
Naturally, no sane criminals would dare to attack anyone in these mafia controlled cities.
But these organizations were nothing when compared to The Mechanicum who had 10 Dark Cities under their rule. The 4 of them had to work together and even by doing that they barely able to contend against The mechanicum.
The Mechanicum did not wanted their cities, if they do, they would have destroyed these four long ago.