We see in Harry Potter and the Deathly Hallows, that Harry and Hagrid, once inside the protective charms set by the Order, were safe from Voldemort. But soon after, at the wedding, we see that Death Eaters break the protective charms easily because the Ministry was fallen. But what kept Voldemort from breaking the charm? Surely he must not have feared capture, not more if he would get a chance to finish his enemy?
The answer given by @AnthonyGrist, Because the Ministry hadn't fallen yet, I agree with. And yes, once he has the Ministry under control, the knowledge of what protection the buildings have been given makes bringing those protections down easier. However, I disagree with the reasoning that he wasn't capable of breaking down the protections even without the Ministry's support.
I believe Voldemort and his Death Eaters could have broken down the protections around the Tonks' home (or even the Burrow). But it would take considerable effort and time to do so. Voldemort and his Death Eaters could constantly bombard the protection until it was cracked open. However, in the time it would take Voldemort and his team to break the protections down, the people inside the Tonks' residence could sound the alarm. This would cause members of the Order of the Phoenix, as well as all the wands the Ministry could muster, to descend upon the attackers and an all out battle would ensue. Even if Voldemort wasn't captured, he would sustain heavy losses to his force. And no matter how strong the leader, it is very difficult to sustain a war without numbers backing you. It is the same reason Voldemort didn't name himself Minister for Magic:
"... Voldemort is playing a very clever game. Declaring himself might have provoked open rebellion: Remaining masked has created confusion, uncertainty, and fear.”
Harry Potter and the Deathly Hallows, Chapter 11: The Bribe
Voldemort waited until he had the Ministry under his control before attacking the houses of Order members. This way, there would be no cavalry arriving from the Ministry to help defend the house under attack.
Because the Ministry hadn't fallen yet
The protective charms at the various Order locations were provided by both the Order of the Phoenix and the Ministry of Magic. It wasn't fear of being captured that prevented Voldemort from removing the protective charms, it was simply the fact that he wasn't capable of doing so.
However, once he had the Ministry under his control, he had access to the power, knowledge and whatever else was needed that only the Ministry controlled to remove them.
I agree with @Anthony Grist (and the implication in the question), it was because the Ministry had not fallen at that time, I'd just like to provide some quotes to support this.
First and most important, at the Death Eaters meeting, Snape says to Voldemort that, as long as the Minitry has not fallen, they won't be able to attack Harry's hiding place:
“At the home of one of the Order,” said Snape. “The place, according to the source, has been given every protection that the Order and Ministry together could provide. I think that there is little chance of taking him once he is there, my Lord, unless, of course, the Ministry has fallen before next Saturday, which might give us the opportunity to discover and undo enough of the enchantments to break through the rest.”
(Deathly Hallows, Chapter 1, The Dark Lord Ascending)
He also says that once the Ministry will have fallen, they will be able to attack that place.
Second, we know that not only the Burrow (Harry's actual hiding place) was given all those protections, but also decoys house, among which the Tonks home:
"So we’ve given a dozen different houses every protection we can throw at them. They all look like they could be the place we’re going to hide you, they’ve all got some connection with the Order."
(Deathly Hallows, Chapter 4, The Seven Potters)
So at the time of the battle of the Seven Potters, the Ministry has not fallen, and Voldemort is not able to break into the Tonks house. Harry and Hagrid are safe as soon as they are in the limits of the protective enchantments.
At the time of the wedding:
- at the begining, the Ministry has not yet fallen and everyone is safe at the Burrow;
- then the Minitry falls: the Ministry enchantments are lift, and the enchantments of the sole Order are broken; Deatheaters attack the Burrow.
In a nutshell:
At that time, the Ministry had not fallen, and the combination of its enchantments and the Order's ones was too strong for Voldemort and the Death Eaters (like Snape foresaw).
The Death Eaters couldn’t be that forceful until the Ministry fell.
What kept the Dark Lord and the Death Eaters from breaking the charm on the Tonks’s house was that they still had to operate somewhat more in secrecy, because they still were in danger of being captured by the Ministry if they were caught. After the Ministry fell, the Death Eaters were able to be more forceful in their methods, since they didn’t have to worry about the Ministry capturing them.
“The Death Eaters got through all those protective charms?’ Harry asked, remembering how effective these had been on the night he had crashed in Tonks’s parents’ garden.
‘What you’ve got to realise, Harry, is that the Death Eaters have got the full might of the Ministry on their side now,’ said Lupin. ‘They’ve got the power to perform brutal spells without fear of identification or arrest. They managed to penetrate every defensive spell we’d cast against them, and once inside, they were completely open about why they’d come.”
- Harry Potter and the Deathly Hallows, Chapter 11 (The Bribe)
With the Ministry then behind them, they were able to be more forceful than they could have been before. They were able to use more powerful spells without concern of being caught or arrested. Also, they could then openly search for Harry, saying he was wanted for questioning in the death of Dumbledore. For these reasons, they could then smash their way through the Order’s protective charms in a way they couldn’t have before they had gained control of the Ministry.