Let me explain why...
First of all his kit and passive are well suited for top lane. He has borderline unlimited sustain, fantastic poke with his Q and Umbrablades.
Its hard to kill him, since- in the event that you CAN kill him, and manage to get onto him, he can disengage off of his fear like nothing happened, and the only thing that's going to get him killed is player error on his end, in the form of overcommitting.
But here's the real reason why Nocturne top is good.
Bot lane is generally the only lane that really matters.
And that is relevant because the biggest strength to playing Nocturne top is, that as soon as you hit level 6. Hard shove your lane(very easy to do as Nocturne). And TP+ult the bot lane while they are lvl 4, and put your bot lane ahead.
this is a nearly guaranteed play, that no one ever really sees coming.