Hell's economy: Hell is ultimately set up to value quantity over quality. Yes, they want the powerful souls of very high-level characters if they can get them. But they're fine making deals with and corrupting less consequential mortals, like NPCs (commoners, experts, aristocrats, warriors), whose souls are just as useful, especially in bulk. The vast majority of these folks are what the FC2 passage is referring to - they have no clear picture of the afterlife, i.e. few to no ranks in Religion or Planes, low Int/Wis due to standard array, few metaphysical concerns that outweigh putting bread on their tables etc. Which leads us to:
D&D settlements and populations: Someone performed a
great analysis of average 3.5 populations using the DMG population tables. From that analysis we can see that in a typical kingdom, you end up with roughly 70% of your population living outside of large towns, 85% outside of cities. That's roughly millions of people in rural areas and hundreds of thousands in urban centers - a world that would look foreign to us today, but one that is ripe for diabolic influence. This is compounded by:
Clerics are spread thin: Especially the Good ones who know the score and are trying to save people; the DMG Web Enhancement that breaks down city demographics further into districts gives us a rough sense of what percentage of a city's population are clerics, paladins, and adepts. These divine servants are woefully outnumbered even in an urban environment, and a good portion of those are either neutral or evil themselves and thus don't really care where their followers' souls end up as long as they further their deities' agenda while alive. And again, that's in a city where you can reliably find and train such classes - in a rural environment, you're lucky if you get a hedgewoman or shaman to tend to the spiritual needs of the people, and Good-aligned divine messengers are even more diluted. Probably less popular too - when the evil ones are preaching things like personal wealth and hedonism, while the boring squares are preaching how we need to share with each other and sacrifice, it's tough to see the majority signing up for the latter and giving the former a wide berth. And even in settlements where you've got a goodly messenger, you've probably got a fiend targeting that place eventually in order to score a win (unless the heroes intervene in time.)