Why do you think these limitations were included by the framers?

-constitution gives power to congress.
-lay and collect taxes, duties and provide for the common defense and general welfare of the united states" - article i, section 8, clause i
-tax to discourage bad behavior (ex. licence for drugs and narcotics)
-taxing power upheld court case veazie bank vs. fenno, 1869

Congress, or the legislative branch, of the United States federal government is the primary elected body of the government and therefore their decisions are most directly effected by the populace of the nation. 

It makes perfect sense to have the authority that is closest to the people making the decisions that have the most effect on the people. IE: taxes and military behaviors.

None of the above :]