Becoming a lesser Lord in Hell meant one could live carefree for centuries, transferring one's torments to the demons below, living off the exploitation of others, without ever worrying about food, drink, or entertainment.
Over time, these Lords grew increasingly powerful and, by carefully avoiding conflict with demons of the same rank, a rigid hierarchy emerged, resulting in the same group of devil lords from hundreds of years ago still in power hundreds of years later.
The main reason was that it was simply too difficult for newcomers to compete. Most demons were immortal, time allowed them to amass power and wealth, which newcomers simply couldn't match.
So, these evildoers weren't suffering in Hell at all; they were clearly enjoying themselves.
And after the Hell Bank was promoted, these Hell Lords were forced into an arms race, then forced to fight, forced to kill each other.