He's late game who likes to hyper farm.
Hyper farming midlaners fell out of favor with the Jungle changes. Their less apt to take Jungle farm inbetween minnion wave cause the razor backs take longer to respawn and the Jungler is less apt to fork them over due to the smite buff.
Instead the Meta Favored Assassin and Play makers who could make effective use of roaming and ganking. But this is why Karthus is still fair, because his ult allows him to AFK gank, cause he can pressure other lanes with his ult, while remaining in mid lane farming.
Karthus is Mana Hungry though so you'll want Blue buff.