In the late 1980s, what caused the rehnquist court to start to restrict congress's ability to regulate various areas of the economy?

changes in the composition of the Supreme Court.  

This was largely due to efforts of President Reagan whose appointments of Justices to the Supreme Court made the Court largely conservative. This includes elevating  William Rehnquist, to the position of Chief Justice in 1986 when Warren Burger retired

One has democracy and the other has dictatorship.

Explanation:

The changes in government such as democracy is initiated after the American revolution without complete revolution and extreme violence while on the other hand, France established a tyrannical government and then a dictatorship under the rule of Napoleon after the French revolution so both revolutions are different one another. Napoleonic Code have in common with the principles of the Enlightenment is that of equality before the law, religious toleration, and the abolition of feudalism. Both ensured that careers would be open on the basis of merit and not privileges of birth.