Because he was cruel and selfish. He didn't care what happened to the valley.
6 seconds sound clip from the Fun and Fancy Free movie soundboard.
You can hear this line at 00:59:33 in the Blu-ray version of the movie.
Quote context
[...]
- Aren't you ashamed of yourself, Charlie? Why do you act like that?
- I don't know. I guess it's just a stage I'm goin' through.
- Well, take off that moustache. Now go over there and behave yourself.
- Yes, sir. Everything I do is wrong.
- But why did the giant want to steal the harp?
- Because he was cruel and selfish. He didn't care what happened to the valley.
- He just wanted someone to sing him to sleep.
- Well, why didn't he turn on the radio?
- Well, they didn't have radios in those days.
- Yeah. That's why they called it Happy Valley.
- What did the giant look like?
- Well, he was... Oh... I'll try and show you.
- He looked something like this.
[...]