Jamieson, Fausset and Brown on 1 Chronicles 15:2

2. Then David said, None ought to carry the ark of God but the Levites--After the lapse of three months (1Ch 13:14) the purpose of transporting the ark to Jerusalem was resumed. Time and reflection had led to a discovery of the cause of the painful catastrophe that marred the first attempt. In preparing for the solemn procession that was now to usher the sacred symbol into its resting-place, David took special care that the carriage should be regulated in strict conformity to the law (Nu 4:5, 15; 7:9; 10:17).