הלכה על סוטה 69:23

It is from the roots of the commandment that it is because the main glory of Israel is the Torah, through which they were made distinct from the other peoples and made into the portion of God. And it is therefore fitting and proper that it should be carried by the most dignified and holy people among us. And there is no need to elaborate on matters that any school child can understand. The entire content of the commandment is elucidated in the verse, and that is that the Kohanim and Levites would carry the Ark on their shoulders whenever they needed to move it from place to place, such as during war or for some other thing; and they would not carry it in a wagon or the back of an animal. And the Sages already castigated David for being mistaken about a thing that even toddlers read about, in his placing the Ark on a wagon (Sotah 34a).