This is a very good question. Written in 1776 near the outset of the American Revolution, the style of the Declaration of Independence reflects the sense of urgency of the time and place it was written. The Declaration of Independence takes a very defiant tone, as it is essentially a list of grievances the colonies had with the English monarch, George III. It does not seek to propose a resolution to these grievances; the only satisfactory resolution ifor the colonies is their cutting themselves from Great Britain's control. In addition, when the colonies declared their independence, they had not actually achieved their freedom; however, the document needed to inspire a sense of confidence that that day would be soon in coming.
The Constitution, though penned only eleven years later, was written in a different time. The colonies had won their victory over the British in the American Revolution. Rather than seeing themselves with only one option as the writers of the Declaration of Independence did, the writers of the Constitution saw possibilities. They were seeking to determine how to formulate an effective government, one that would have neither the same pitfalls of colonial governments, nor share the more tyrannical elements of British occupation policy. The Founding Fathers show a different sense of urgency in the Constitution. They had moved past British control and needed to replace that memory with something that reflected American interests and concerns for government. This resulted in a document that is assertive like the Declaration of Independence but also a document that lacks the sense of defiance that the Declaration of Independence projects.