All from my perspective...
It really comes down to how much power the followers have to draw upon and how and, potentially, if a Demon Lord could care less about it's followers.
A god would be the power source for its evil cults clerics/priests/etc. If that gods followers are the source if its power then it will be concerned with its followers. If its source of power is not its followers (and rather a concept, such as Hate, the power being generated by all the hate in the world) it may not really care about them much. Lastly, is it possible to completely kill a god in your game?
With a demon lord, is it the source of the cults spell casting followers power? I would almost always say no. As such the demon lords followers would not have as much power to draw upon and the demon lord would not care about the welfare of its followers. Lastly, it almost surely is possible to kill a demon lord in most peoples games.