Because it deals with people, economics is a "soft science". The science is in the formulas and mathematical principles which can analyze trends and give some predictions for the future based on policies.
However, it cannot be separated from theology.
This comes into play with ideas like morality (what activities are right and good and should be encouraged), teleology (purpose), and anthropology (theory of man).
One of the big assumptions is that people are rational when it comes to economic decisions.
This ignores the fact that sin is irrational.
I recall after one of the recent bubbles, Alan Greenspan reported that he was surprised that greed could cause people to make bad decisions.
On the opposite side, there are those who believe that any notion of property, or that some might have more than others is inherently evil.
I am not so much interested in the ideal, as what Christianity has to say about economics in the presence of sinful actors.