This is a good question, and as far as I know, not one that's been answered directly, in the books or by JKR, making it a bit of a gap in the canon. However, this is my speculative answer.
Voldemort needed a loyal servant to help him regain his body. Wormtail only escapes at the end of Azkaban, and I think we can assume it takes him a while to find his master. Therefore, not much time to hatch a plan to get Harry on his way back to the Dursley's. This leaves two potential "commute attacks" - at the beginning of GoF, and at the end (unless you leave it until OotP, which would make GoF a rather uneventful book).
At the beginning of GoF, Harry is staying with the Weasleys. They take a taxi to King's Cross, and at this point, Harry is with a bunch of competent wizards (Mr and Mrs Weasley, Bill, Charlie, etc). Not a good attack point. This leaves only the one commute, at the end of GoF.
So, Voldemort could have waited a whole school year, and charged Pettigrew and Barty Crouch Jr with a Dursley ambush. There are a few reasons why I think he may not have done so.
- Ignorance: We don't know that Harry is protected enroute to the Dursleys, as you've pointed out. However, Voldemort would have little reason to assume he is not. He is not necessarily aware of how Harry gets to and from Privet Drive, and might think he gets there through any number of magical means (assuming that the Dursleys wouldn't bother to pick him up is, after all, a reasonable assumption). He knows that Dumbledore is an extremely bright wizard who would be unlikely to leave his charge unprotected.
- Logistics: Voldemort, at this point, is a weak little thing wrapped up in a cloak. Let's assume they wouldn't do the whole potion/cauldron ritual at the side of the motorway - they would need to get Harry somewhere fairly isolated, like the graveyard. We can assume he had two available helpers (Pettigrew and Crouch). One would need to stay with Voldemort, leaving one of them to attack/kidnap Harry. However, this could be quite dangerous - Harry is pretty good at responding to random attacks, and they wouldn't be able to kill him (they need him alive, as least for a bit). He'd also have his wand with him, and even if the kidnapping succeeded, he'd arrive at the graveyard exceedingly alert and ready to fight. (Granted, there would be ways around this, by knocking him out and such.) We might think that Voldemort wanted a solution that would take Harry by surprise completely - he was already expecting the cup to be a Portkey (as was everyone else). It also fits in with making people disbelieve Harry - a stressed out boy at the end of a media-heavy year-long Tournament is less likely to be believed, as opposed to the Boy Who Lived getting snatched in front of his muggle family.
- Impatience: He wants to be doing something to get his body back, and doesn't want to wait for a whole year.
- Superiority: A recurring theme in the books is Voldemort's reticence to accept that there are things more powerful than magic. He doesn't seem like the kind of person who would "stoop" to admitting that he couldn't attack Harry in the wizarding world and would have to settle for a roadside attack. I suspect he would want to show off his power and cunning, which sending accomplices to a road they're unfamiliar with would not accomplish. By getting Barty Crouch Jr/Moody to infiltrate Hogwarts, and hijacking the Triwizard Tournament, he gets a trusted spy in Hogwarts with constant news from the Wizarding World and the teachers, gets the trust of Harry (who does trust Moody, for a bit), and uses a variety of magical means to get what he wants (Harry in the graveyard). Although, like you say, there is a lot that could go wrong with this plan, I suspect a magical plan would have been far more attractive to Voldemort than admitting that he couldn't attack Harry in a magical place.
There's a comment, and a previous answer that was deleted, suggesting that it has something to do with the magical protection afforded to Harry by his mother's sacrifice.
The comment suggests that the protection extends from the Dursley's house to their car.
I guess it's possible, but it seems a bit far-fetched. Harry has the standard protection given to him by his mother's death regardless of where he is, but the further protection spells that Dumbledore placed, seem to be placed only on the house. The difference between Lily's sacrifice and Dumbledore's enchantments is explained in this excellent answer by Joe White. For the car to have the same enchantments as the house, Dumbledore would have had to extended the spell specifically to the car. This would seem an odd decision - after all, he's only trying to protect the house. Protecting Harry whenever he leaves, for a walk, drive or train journey, is far more tricky.
The previous answer suggested that his mother's protection extends to any location as long as he still calls Privet Drive home, so he would have been protected.
He was protected, but the comment about calling Privet Drive home refers to Dumbledore's spells on the house, not his mother's protection on him (again, see Joe White's answer). His mother's protection protects Harry wherever he is - we know this. However, it also means that he had the same protection when he was at the graveyard - which didn't stop Voldemort's plan.