Because they should be more than just generic EVIL PLANAR GUYS who happen to be in a war against each other. Yeah, yeah, there's three metric tons of earlier edition splats which go into history, culture, organisation, personalities and shoe size detailing just how demons and devils are different. But then your character shouldn't know that stuff either, right?
I can see that if you were confronted by some misshapen, multilimbed monstrosity with claws and fangs and drooling spittle from its gigantic maw, you might not know which particular demon it was. But the first instinctive thought for most people on seeing something like that would be "it's a demon" and that's something which is quite reasonable. There's no reason, whether from an ingame or out-of-game PoV, to start playing games like "OTOH, it could be a devil...".
The greatest point of difference, really, between demons and devils as they stand right now is that one side is CE and the other LE. Since it appears that they're going to be deemphasising the role of alignment, that no longer becomes sufficient. So there's a need to find other ways to make them mechanically and thematically distinct from each other.