Why does the Torah ascribe the construction of the Aron to Betzalel?
Rashi: Because he put into its construction more than all the other Chachamim, it is called after him.
Ramban and Seforno: Because he actually constructed it. Refer to 36:8:1:1.
Da'as Zekenim: His name hints to "b'Tzel Kel", that he knew by himself (Ru'ach ha'Kodesh) to make the Mishkan before the Aron [and other Kelim]. Also, Tzel Kel (the shade of Hashem) dwells in the Aron (Rosh).
Moshav Zekenim, Ba'al ha'Turim (38:1): He was in the shade of Hashem and knew the secret of the Aron and Merkavah, that the Aron corresponds to the Kisei ha'Kavod.
Why did the Torah specify "Atzei Shitim"?
Da'as Zekenim: In the future, they wll sin at Shitim (in Arayos and idolatry - Bamidbar 25:1-3), therefore Hashem established [the Aron of Atzei Shitim] to atone for the episode in Shitim. Rosh - Hashem prepares the cure before the illness!
Why was the Aron the first Kli of the Mishkan to be made?
Rosh, Da'as Zekenim: It says "Pesach Devarecha Ya'ir" (Tehorim 119:130) - light was the first creation, therefore the Aron was made first, for the Torah, which is called light, is put in it - "Ner Mitzvah v'Sorah Or."
Where were the Luchos before Betzalel made the Aron?
Riva, from Rashi Devarim 10:1: Hashem told Moshe to build an Aron before giving the second Luchos (34:1). After Betzalel built the Aron for the Mishkan, the Luchos were put in it. Moshe's Aron used to go out to war. Betzalel's Aron went out only once, in the days of Eli, and they were punished, and it was captured.