Why were Thomas Jefferson and James Madison opposed to the idea of creating a national bank?

They were a) concerned about the kind of power that would give the federal government over the money supply and b) the believed it to be unconstitutional, as "the creation of a national bank" is not one of Congress's enumerated powers.

Jefferson once commented that:

"I believe that banking institutions are more dangerous to our liberties than standing armies. If the American people ever allow private banks to control the issue of their currency, first by inflation, then by deflation, the banks and corporations that will grow up around [the banks] will deprive the people of all property until their children wake-up homeless on the continent their fathers conquered. The issuing power should be taken from the banks and restored to the people, to whom it properly belongs."

The same would hold true for a bank created by the government, as democracy is its own worst enemy. Of course, this could just be him being dramatic without cause, which he sometimes made a habit of.