The simple reason is, that the higher you get the more important the trashpulls are. Fractals on high difficulty are pretty much only about thrash & trash events where Black Powder is just the best skill a thief has (you can substitute with S/D & blind on stealth but it's not as good as S/P). Let me paraphrase (only to get slaughtered by Tolkien Fans):
"A hundred Blades to rule them all, Black Powder to blind them, Into the Void to bring them all and a Ring of Warding to bind them."
For 2 Warrior, Guardian, Mesmer, Thief at least .
That's not to say the bosses can't kill you, they can... but the trash takes time, and even the (probably) hardest boss in fractals, the ascendend grawl shaman is all about trash.