He couldn’t just make anything a Portkey. He wanted ONLY HARRY in the graveyard. If he turned any random thing into a portkey, how could he know that no other student would touch it? Even in his office – there have been numerous times where students sneaked into teachers’ offices and touched/taken things they weren’t supposed to. So it had to be in the maze.
It would be a lot easier to slip him a portkey while Harry was in his office, and a lot more guaranteed than Harry winning the cup, which took a tremendous amount of effort, and wasn’t guaranteed.