1)
Why did Shlomo gather the Chachamim and the Nesi'im?
1.
Malbim: To be Mekadesh the Azarah, the Sanhedrin of 71 is needed. These are the Chachamim that he gathered. He gathered also the Nesi'im for the honor of the king and the Aron.
2)
Why was the Aron in Ir David?
1.
Rashi: David brought it to there when he took it from Beis Oved Edom (Shmuel II, 6:12).