Because said shows that get put out either had:
a) A long time on the making therefore a dedicated fanbase (Berserk, JoJo, Hokuto no Ken, Devilman) and even then they have a hard time to get a proper adaptation (Miura has said that the reason Berserk never had much anime is simply because studios don't see much faith in a franchise like that being profitable; and prior to the anime, JoJo was originally supposed to have all parts in movie form by Studio APPP starting with Phantom Blood but we all know how that turned out)
b) Newer shows with darker stuff often resort to areas where it can attract fanservice (Goblin Slayer, god fuck that show, has some harem anime to it considering there's a bunch of different types of girls wanting the MC's dick)
c) Often at times they're exclusively marketed for their violence (Terra Formars, Attack on Titan)
RWBY's kinda on this gray zone where they can't really find what to sell on it; since V4 and 5 are painfully slow with not a lot going on?
Mature themes? Nope
Violence porn? Nada
Stuff to suggest/lewd/fantasize about the main characters? Definetly a nope given RT's stance on this stuff