If Mad-eye Moody's only purpose was to get Harry to Lord Voldemort then why did he have to wait until the very end of the Tri-wizard tournament? Being a teacher he had Harry at his mercy anytime so should he take the pain of getting Harry into the tournament, risk helping him with the tasks, and so on? He could have easily done it any time during the entire year with any other object as the port key. Why was the Triwizard cup chosen? Is there anything that I'm missing?
Barty Crouch Jr said it himself, in that he couldn't do ANYTHING to risk Dumbledore suspecting him. Let's face it, you have to be pretty damn conniving to get something past Dumbledore.
Anyway, it's just a story. If he did it your way, the book would be about 100 pages long.
It's all about the journey :)
It was Voldemort's plan. Who was Barty Crouch Jr. to decide to do something different from what Lord Voldemort decided?
As for why Voldemort wanted to wait - he had detailed plans to orchestrate. And enough patience/intelligence to follow them at appropriate pace.
Part of those plans was making sure that he didn't announce his resurrection to the Ministry right away, as abducting Harry via Portkey from Hogwarts would - instead, had he disappeared from inside the Maze, nobody would have known what happened, or blamed Moody/Barty.
There are three critical components to consider.
One: portkeys are not supposed to be used to transport oneself onto or out of the grounds of Hogwarts without prior knowledge of the ministry and Dumbledor. It is possible to break this rule (as we see Dumbledore do himself in Order), but as the only example of a wizard doing this without ministry permission is Dumbledore himself, Crouch probably wasn't capable of it. The cup was an "allowed" portkey and was supposed to transport the winner out of the maze and back to the stands to be celebrated. Crouch Jr. altered it a little before placing it by adding the portus to the graveyard. This is simpler than layers of spells probably needed for circumventing portkeys that have not been "registered" so to speak.
Two: While Crouch could have killed Harry and then escaped, Voldemort wanted to kill Harry himself and he needed Harry's blood. It hurt Voldemort's pride that he had been defeated by a baby and he felt he needed to prove there wasn't really anything special about, "the boy who lived." He talks about this need in "The Death Eaters" when he says:
"You know of course, they have called this boy my downfall. . . but no matter, I can touch him now! [Harry feels terrible pain in the next paragraph while V touches him] I miscalculated my friends, I admit it."
He goes on for quite some time re-hashing stuff we really already knew from the first three books. He talks about the Sorcerer's Stone and being thwarted again by Harry and his dissappointment in the lack of a search for himself by the Deatheaters standing there, but goes on and begins to speak about Bertha jorkins and how the plan to capture adn kill Harry evolved,
"But how to get at Harry Potter? For he has been better proteced than I think even he knows, protected in ways devised by Dumbledore . . .
as long as he is in his relation's care. Not even I can touch him there. . . Then, of course, there was the Quidditch World Cup. . . I thought his protection might be weaker there, away from his relations and Dumbledore, but I was nt yet strong enough to attempt kidnap in the midst of a horde of Ministry wizards. and then he would return to Hogwarts . . ."
"So how could I take him? Why by using Bertha Jorkins's information of course. Use my one faithful Death Eater, stationed at Hogwarts, to ensure that the boy's name was entered into the Goblet of Fire. . . "
Voldemort goes on to have Wormtail release Harry so they can have a "duel" in the next chapter.
Additionally, Harry neede to be portered to Voldemort at an expected time and a particular place. There was a complicated bit of spell work and a potion involved here so just having Harry show up when ever crouch could happen to get him to touch the proper pen or whatever wouldn't have worked well.
Just before having Harry released for the Duel Voldemort says:
"You see, I think, how foolish it was to suppose that this boy could ever have been stronger than me, but I want there to be no mistake in anyone's mind. Harry Potter escaped by lucky chance and now I'm going to prove my power by killing him here and now. . ." Emphasis mine.
Sure, Voldemort could have had Pettigrew call his Deatheaters to guard the graveyard and wait and capture him and then had pettigrew start in on the potion etc. etc. but then, I don't think Voldemort trusted the other Deatheaters to let Pettigrew go through with it, he certainly didn't want all of them seeing him in his weakened state and doing it the way he did meant he could put on a show and monologue for awhile - something all Villians truly enjoy.
If everything had gone according to plan, big V would have been able to use the Portkey to get himself and his death-eaters into Hogwarts (undermining Dumbledore's ability to protect anyone). No doubt parading the lifeless body of "the boy who lived". This would have offered the maximum amount of worldwide exposure - as the event was attended by wizards from all over the world. It would have also given him the opportunity to kill the heads of three major eurpoean schools, the Minsiter for Magic and I can only assume other influential figures in the magic community. Taking all this into consideration the Port Key AND the Tournament could be considered a dream scenario for a large-and-in-charge return.
Personally, I can't be sure if this level of thought went into this area of the story. After all the plan was destined to fail purely because it was Book 4 in a series of 7, but this is what I plugged the plot hole with so I could sleep at night!
Edit: This is supported by the fact that the Portkey was configured to return to Hogwarts at all. I don't think this is the default behaviour for them.
Another point is that Voldemort wanted a show. He wanted to make a grand event of Harry's kidnap, right under Dumbledore's nose.
He does not just want Harry, he wants him a glorious way which will have the wizarding world talking in whispers about it for years. Kidnapping a young teenager in the middle of the night is a feeble way to go about things for the great Voldemort.