I'd like to go a bit deeper into Richard's comment on his answer about being under pressure and on how Rowling constructed the scene.
In that particular scene, there's this very telling paragraph about what's going on in Harry's mind.
...Harry remembered,...the dueling club at Hogwarts he had attended briefly two years ago.... All he had learned there was the Disarming Spell, "Expelliarmus".
GoF, Chapter 34, p660. Scholastic Edition.
So from the onset, Expelliarmus filled Harry's mind. He did not intend to do it. At the time, he was not taking moral high ground that he's no Voldemort so he won't kill. It was instinct that made Harry use Expelliarmus.
Personally, I think this is where Expelliarmus became his signature spell. I could go into all my theories about what's going on in Harry's mind between the lines but that's out of place here so I'll just leave you with a personal real-life observation that mirrors this reflex from Harry.
I trained in Taekwondo and I noticed that (applies even in my case), the first time white-belts spar, they don't use the good techniques they are supposed to know but the most basic one. It does not give the most points nor is it KO-quality. Most are not aiming to win. Nervous as they are, they just want to retaliate enough to survive the round.
Very same conditions with Harry, if not more intense. Note that he has been using Expelliarmus since his second year. He has used it in his third year against grown-up wizards (Snape and Black). Compare that to Stunning which he only learned not too long ago for the third task. It's not far-fetched that faced with intense pressure from another more-powerful wizard, he'd stick to what experience has proven effective.
TL;DR Harry was not trying to save the day and kill Voldemort. He was acting out of survival instinct so he fell back on Expelliarmus.