Why do you think each state has its own constitution or governing document rather than simply following the rules outlined in the u. s. constitution?

Because the U.S. Constitution is very broad and it does not detail every aspect of the taxation and debt regarding each state.  

Explanation:

The State Constitution is followed by the states because it outlines in a more detailed way the taxation and debt that can be incurred by each state. Usually the state constitution is almost the double long to the country constitution.

The first state to write their own constitution was Virginia in 1776. Afterwards each state wrote their own constitution.

the us constitution is a living document, meaning that it can be changed over time as the country changes.