Because being forced to use such a low damage weapon at lvl 90 just sucks.
A new lvl 90 TH+1 dagger won't help because we'd be forced to use that *and* the Thief's Knife.
Whatever it is, make it very hard please. I'd like the Thief's Knife to mean something again. Turning the Thief's Knife into a trash item by making it a 100% drop was one of the worst ideas ever. It would have been better to just make it Rare/Ex.