1)
Why does it say "va'Yivchar... Bi"?
1.
Metzudas David: Even though Hashem rejected me from building the Beis ha'Mikdash due to the blood that I spilled, He did not despise me. He chose me to be king as long as I live!
2)
Why does it say "Ki vi'Yhudah"?
1.
Malbim: There were three choices. Hashem chose David's Shevet, his house (family), and him.
