What’s the role of ‘because’ in this conversation?

I’ve lost track of the logical flow of the following conversation between Lucius Malfoy and Harry.

And still, behind his back, Dobby was pointing, first to the diary, then to Lucius Malfoy, then punching himself in the head.

And Harry suddenly understood. He nodded at Dobby, and Dobby backed into a corner, now twistng his ears in punishment.

”Don’t you want to know how Ginny got hold of that diary, Mr. Malfoy?” said Harry.

Lucius Malfoy rounded on him.

”How should I know how the stupid little girl got hold of it?” he said.

”Because you gave it to her,” said Harry. “In Flourish and Blotts. You picked up her old Transfguration book and slipped the diary inside it, didn’t you?” (Harry Potter 2 [US Version]: p.336)[Bold font is mine]

N.B.: Dobby is a kind of servant of Lucius Malfoy, Harry’s enemy. He conveys the fact by gesticulating that Lucius is the culprit who gave Ginny a kind of jinxed diary. He punishes himself for the betrayal of his master.

To be honest, I can’t understand why I can’t understand. I was thinking it was because I didn’t understand “Don’t you want to know -”, and asked. But I still can’t get why Harry is saying “Because -”. It seems that they have a talk in a secret code.

How does the conversation progress? What reason is Harry conveying by saying because? I’d be glad if you could guide me through the conversation.

Solution 1:

In this, "Because" is a sort of abbreviation for "You would know this because".

Most languages, as spoken by native speakers, get shortened. This often causes confusion to new learners, because they don't have enough context to understand why a particular word in a particular place could mean what it does.

"Don't you want to" could be replaced by "Do you want to", or "Would you want to", or even "Would you like to", and have the same basic meaning. There would, however, be a subtle difference in tone (a little like different politeness modes in Japanese), and also different styles.

Equally, "Because you gave it to her" could be replaced by:

"You gave it to her"

"The reason is you gave it to her"

"Reason is, you gave it to her"

"Reason: you gave it to her"

Solution 2:

I think the following conversation would have pretty much the same meaning.

"Aren´t you curious about how Ginny got hold of that diary, Mr. Malfoy?" said Harry.

Lucius Malfoy rounded on him.

"How should I know how the stupid little girl got hold of it?" he said.

"By you giving it to her."

"Because you gave it to her" is perhaps just a little more accusing way of saying the same thing. Hope that helps. Please note that I´m a newbie on this page and not a native English speaker.

Edit: actually the anwer above mine made me understand this better.

"...how the stupid little girl got hold of it?" could be answered "by you giving it to her"

but

”How should I know...?" is the actual question answered here by "Because you gave it to her"