Why does it say "Az Amar"?
Vilna Gaon: Before, he thought that now we may carry it via a wagon (refer to 13:7:1:3-4). After Hashem killed Uza, he realized that only Leviyim should carry it ? forever.
Malbim: Before, he thought that Yisraelim may carry it, not via poles, and we may put it on a wagon. Now he realized that Leviyim must carry it [via poles].
Why does it say "Ki Im ha'Leviyim Ki Vam Bachar"?
'Rashi': Hashem chose that Leviyim should carry the Aron, and not a wagon. This is why Uza was punished before - Ki lemiba'Rishonah? Lo Dereshnuhu ka'Mishpat" (verses 12, 13).
Are all Leviyim proper to carry it? The Torah designated Bnei Kehas!
Malbim: All Leviyim are qualified ? "Hivdil Hashem Es Shevet ha'Levi Laseis Es Aron? Ad ha'Yom ha'Zeh Al Ken Lo Hayah la'Levi Chelek" (Devarim 10:8-9).
