The difference is that the impact of deciding that the ball did, in fact, cross the touchline has far less bearing on the outcome of the game than deciding that the ball did cross the goal line.
While we're on the subject - the whole of the ball has to cross the whole of the line. The number of throw ins/goal kicks/corners given when that criterion hasn't been satisfied winds me up.
I know that, but there is nothing in the laws to say that that means there is a higher standard of proof when deciding whether or not a goal has been scored.