In Parashat Naso, we read about the logistics of dismantling, transporting, and reassembling the Mishkan as the nation wandered through the desert.
The Cohanim were responsible for this task, and the tribal leaders donated wagons to help with the transportation. However, the Kehat family, who were responsible for carrying the most sacred items such as the ark, were not given any wagons.
Instead, they were instructed to carry these sacred items on their shoulders.
This law of carrying the ark by hand, rather than by wagon or any other means of transport, was not just for the time of the desert wanderings. It applied throughout Jewish history. In the book of Samuel, we learn of the death of Uzzah for accompanying the ark on a wagon.