Answer
One aspect of the economy that causes the lags in the effect of monetary policy is that firms and households set their spending plans in advance. Something else that causes a lag is that, for fiscal policy, there is a lengthy political process that governs changes in spending and taxes.
Work Step by Step
The implication of the set spending plans for monetary policy is that the changes in monetary policy have minimal effect on aggregate demand for up to six months after the change is made. The implication of the political process on fiscal policy is that policymakers need to look ahead and forecast the economic conditions that the change will enter. (Also, the economic forecasting is an imprecise science, so the conditions could be completely wrong.)
