It's stupid how Snape didn't resist Nagini when Voldemort ordered Snape's death.
TL;DR: To extend the plot-line
Okay, so that isn't the complete answer, but has a lot to do with it, which is the same reason Voldemort had Nagini attack Snape, versus using the Avada Kedavra curse to kill him instantly.
As we know, Nagini's attack did not instantly kill Snape, which allowed Harry to collect Snape's memories, and fill in the missing pieces to the "How to Kill Voldemort" Puzzle left by Dumbledore. It can be argued that, without knowing that a part of Voldemort lived within Harry himself, that Harry never goes to the forest and sacrifices himself (he may go to the forest, but choose to attack Voldemort instead). So for that reason, it is essential that Snape not die instantly, so he can pass on this essential information to Harry.
To more directly answer your question on why Snape didn't resist, it is rather simple. He didn't have time. Snape did not have his wand drawn at the time that Voldemort ordered Nagini's attack, Voldemort did have his wand drawn (he had to release Nagini from her enchanted cage). The way it is described in the book is that Nagini attacked too quickly for Snape to be able to draw his wand and defend himself. Furthermore, it is not explicitly stated that Voldemort ordered the attack in English. He could have ordered the attack in Parsletounge, in which case Snape would not have realized that Voldemort had ordered Nagini to attack until after the snake had already begun to attack. Finally, it is likely that if Snape did make an attempt to draw his wand to defend himself, or to turn and run, that Voldemort (remember, with wand in hand) would have cast a curse to prevent Snape from doing so. So in the case that Snape did have enough time to resist, he is smart enough to realize that his chances of delivering the message to Harry, while slim, are still greater by not resisting, than by resisting.
In addition to the link provided in the comments and the answer above, in no sense is it true that "Snape didn't resist Nagini when Voldemort ordered Snape's death."
'My Lord!' Snape protested, raising his wand.
'It cannot be any other way,' said Voldemort. 'I must master the wand, Severus. Master the wand, and I master Potter at last.'
And Voldemort swiped the air with the Elder Wand. It did nothing to Snape, who for a split second seemed to think he had been reprieved: but then Voldemort's intention became clear. The snake's cage was rolling through the air, and before Snape could do anything more than yell, it had encased him, head and shoulders, and Voldemort spoke in Parseltongue.
There was a terrible scream. Harry saw Snape's face losing the little colour it had left, it whitened as his black eyes widened, as the snake's fangs pierced his neck, as he failed to push the enchanted cage off himself, as his knees gave way, as he fell to the floor.
Harry Potter and the Deathly Hallows - p.527 - Bloomsbury - chapter 32, The Elder Wand
So he certainly did resist. Not to mention the entire conversation which preceded this scene in which Snape tried everything to get Voldemort to allow him to leave and seek out Harry Potter.
(By the way, to add to bz032002's answer, it's clear that Voldemort certainly did give the order in Parseltongue. Although by that point I don't think Snape was in any doubt about what was about to happen to him.)
Now, by the time he was trapped in the enchanted cage, it was too late. He could not escape the cage and he had no chance at all to manoeuvre into a position and kill a massive great snake that instantly attacked and killed him.
He could hardly have attempted an attack on Nagini before, for the same reason Harry couldn't - there was a great big enchanted cage and, to paraphrase Snape himself, do you disagree that attempting to murder his favourite snake might have turned Voldemort against Snape? I think Voldemort would probably have noticed and killed him. But, more to the point, I don't think Dumbledore ever explained the importance of Nagini's death to Snape, so he'd have had no reason to attack the snake until it became clear the snake was to be his killer - by which time ...
The only option really available to him was to duel Voldemort before he had a chance to set Nagini on him - in those few moments between Snape raising his wand and Voldemort swiping his through the air. But by the time it dawns on him he's about to be killed, he's not got a lot of time to think. He's still hoping to get out of there alive and be allowed by his master to seek out Harry Potter. You don't exactly take on Lord Voldemort unless you can help it - Snape knows his best chance is to try to talk Voldemort into letting him leave to give Harry Dumbledore's message. I'd say it's pretty natural - what he did. Sharp words of protest while raising his wand in defence. The most powerful Dark wizard of all time then speaks and so he listens, hoping for a way out. And then the wand falls and he's already dead. Besides, we know how quick Tom Riddle is - as if even Severus Snape stood much chance of completing any spell in time. No, of course he was still hoping to be allowed to leave - putting his hope in words.