Why did Moshe make thee Aron specifically out of Shitim-wood?
1.
Oznayim la'Torah (citing the Tanchuma): Because the Aron that Betzalel made was made of Atzei Shitim to atone for the sin of Ba'al Pe'or, which (Hashem knew)
1
was destined to take place in Shitim in the fortieth year.
2
And evidently, so did Moshe - seeing as nobody told him to make the wooden Aron out of Shitim-wood.
2)
What are the implicatons of "ka'Rishonim"?
1.
Targum Yonasan: It means that the measurements of the second Luchos was equivalent to those of the first ones - each Luach measured six by six Tefachim by three Tefachim thick, and that they were also made of saphhire.
