Why didn't Voldemort kill Petunia

Why didn't Voldemort murder the Dursleys?

The Dursleys were safe by the same kind of magic that protected Harry. However, all were only safe within the confines of their home until Harry came of age (i.e. he turned 17). Harry or one of the Dursleys could have been attacked outside of their home. When Harry was approaching his 17th birthday, the Order of Phoenix moved the Dursleys to a safe place. That night, Harry and his guard were attacked as soon as they crossed the 'border' that protected the house, as they could not be attacked within that border. It was the house that had protection.

Harry Potter and the Half-Blood Prince:

Chapter 3: Will and Not

..but Dumbledore silenced his finger, a silence that fell as if he had struck Uncle Vernon to silence. “The magic I created fifteen years ago means that Harry has strong protection while still being able to call this house 'home'. No matter how miserable he was here, how unwelcome, how badly treated he may be, you at least reluctantly allowed him into the house. This magic will stop working once Harry turns seventeen; In other words, right now he's becoming a man. All I ask is that you allow Harry to return to this house before his seventeenth birthday to make sure the protection continues until then. "

Harry Potter and the Deathly Hallows:

Chapter 3: The Dursleys Go

"You claim," said Uncle Vernon, and began to pace again, "that this Lord Thing -"

"- Voldemort," said Harry impatiently, "and we've been through this about a hundred times." This is not a claim, it is a fact, Dumbledore told you last year and Kingsley and Mr. Weasley - "...

... "- Kingsley and Mr. Weasley explained all of that too," Harry went on relentlessly. "As soon as I'm seventeen, the wards protecting me will break and expose you and me ..."

Harry Johnston

It's not just in the house itself. At the beginning of Deathly Hallows says Moody, "the spell will break once you are out of its reach," so clearly it has a range, not just "inside the house". How far away it is is not clear, but from the following scene we can see that it is far enough away that Harry could no longer see Dursley's house.