What was the main purpose of the Constitutional Convention at Philadelphia in 1787?

To fix the flaws in the Articles of Confederation.

Their assignment was to fix the flaws in the Articles of Confederation, the document that governed the U.S. at the time. But it quickly became evident that the Articles were too flawed to be fixed, so they resolved to replace them, even though this was not their assignment. The result is the U.S. Constitution.