A main economic cause of the new imperialism of the 19th century was

the need for raw materials for industrial production.

Explanation:

The development of machinery due to industrialization made European society able to produce goods at significantly faster rate compared to other societies at that time.

One downside of this is that they couldn't provide enough raw materials to sustain the overall production.

Because of this, they developed a plan to seek new natural resources from other countries. This motivate them to conquer weaker countries and took control over their resources in order to accommodate their production (new imperialism eventually started)