I think it is also just a natural "feature of the terrain". Criminals need to communicate with their customers and each other to coordinate, but they cannot use

I think it is also just a natural "feature of the terrain". Criminals need to communicate with their customers and each other to coordinate, but they cannot use "normal" apps because those can be presumed to be compromised by the police. This creates a natural funnel where criminals are driven to these custom apps, similar to how old-time armies would fight over things like river crossings and mountain passes because the opponent had no choice but to go there if they wanted to invade at all.