When you become more aware of Karthus' passive and how powerful it is, and how his kit performs when you just stop caring about your HP, you start to lean on it. You bask in the double/triple kills you get all game. You fling your champion into death again and again. Until your K/D is 18/25.
I've seen it time and time again.
When you get good at Karthus, you actually stop caring about winning (even if you keep it in your mind and try to push it to the front) and more about reveling in as much carnage as possible.
In a lane like top, where you aren't up against as many squishies, Karthus can't rely on the cheese elbow room nearly as much.
You have to start playing him like a normal ranged top laner, which goes against Karthus' very being and how people like to play him.
So there's a bit of a dissonance that can create a pitfall for even experienced Karthus players
That's my two cents