And that's because he's a ticking time bomb. Normally, if you're in an unfavourable match-up you can just sit back, play more cautiously, just farm and focus on surviving. Maybe you give up some cs in order not to get killed, but that's not the end of the world.
But if you do that against Nasus, you know what will happen. Eventually he'll have enough both damage and tankiness that he can just take all your towers and there's nothing you can do about it. And if you try to stop him, he'll just press R and two-shot you.
So you simply have to shut him down before that happens. But how? What if you lack the tools to do that - you don't have the damage to kill him outright and your poke only tickles him? And what if you don't manage to do it? It feels like all you can do is ff.
Also, his design is so dull. He can grow immensely powerful without even interacting with his lane opponent. I.e. he gets fed by simply pressing Q. Nasus player only faces one decision in the laning phase: should I farm or should I farm?
I won't even mention his abilities that are designed purely to frustrate the shit out of his opponent.
And don't tell me that my jungler can gank him, because I'm talking about things that I can control. If all I can do is pray I'll get lucky enough to deserve jungler's attention, then I might as well play fucking roulette.