Here's why:
* Metronome is a move that randomly selects a move from the entire movepool. This means the move it uses is not a set move known by the user.
* Sketch only allows Smeargle to learn a move that the opponent has used in the current battle. It can't learn moves that are not actually known by the opponent.
Since Metronome's move selection is random and not a move the opponent "knows," Smeargle cannot learn it with Sketch.