Why Uzzah Died

2 Samuel 6:7  And the anger of the LORD was kindled against Uzzah; and God smote him there for his error; and there he died by the ark of God. 

David had a heart that sought YHWH and wanted to bring the presence of YHWH into Jerusalem. His motive was right, but his attempt failed for three reasons, namely,

1) David did not use the God-instructed method of moving the ark, that is, that the ark was to be borne on the shoulders of four anointed priests of the sons of Aaron, but instead (probably ignorantly), he used the Philistine's method of transporting it, in a cart drawn by cows. God's ways are better than man's ways, and the Philistines got away with it because they were ignorant, and were not God's people, but God required His people to whom He had revealed His law to follow His ordinances. David, as a king, was required by YHWH (Deut 17) to copy out the entire Law of Moses, to learn it and know it.

2) The Ark of the Covenant carried the very holy presence of God. The box was nothing special, but because God's presence dwelt in it, it must be treated with reverential fear. God Himself was probably not revered by the people in the way He should be, owing to years of backsliding as a nation, and the irreverent, apostate priesthood as shown in Eli and his sons. David perhaps did not realize just how much God was, is, and is to be feared and how holy and pure He is.

David had not only failed to fully follow God’s commands and to realize the severity of God’s holiness, but he failed to instruct the men transporting it and impart to them the right principles and attitudes with regard to it. Uzzah was the son of Aminadab, and the ark of the covenant had been in his father’s house for quite a time. Uzzah had good intentions, to keep the Ark from falling. However, proper preparation of heart, mind, and body, were needed for such an important task. This was a hard lesson David learned. The next time, he with utmost care and precision, in most reverent of fears, followed God’s way of bringing the ark in Jerusalem.