This was due to the dominant economic philosophy during that time period called mercantilism. Its doctrines state that by achieving a material superiority therefore, if a country manages to maximize its exports over the level of imports, it would have found the main source of wealth.
The positive side of such an approach was the superb increase in the trade relations overseas, when colonist metropolis started trading with their colonies. The negative point was that this trade was polarized. On one hand, the colonies were intensively exploited for their raw materials which they sold at a cheap price to their metropoli. On the other hand, the metropoli exported much more expensive final products manufactured by the raw materials imported, therefore they always achieved the desired trade surplus (exported a larger value than was imported) and never let colonies do so. Metropolis gain wealth at the expense of colonies that could never reach in many cases their top potential in this mercantilism model.
