Considering how early intervention programs to improve intellectual Development have some mark improvement in the IQ of participants it can be inferred that the main reason children from low SES families earn lower scores on IQ test is that

the parents have difficulty providing an intellectually stimulating environment for their children.

Explanation:

SES stands for socioeconomic status.

Parents form high Socioeconomic status are more likely to have money to spend for the development of their children, while parents from low Socioeconomic status barely have enough money to survive.

Because of this, children form high economic status are more likely to grow up with various toys and entertainment that can stimulate brain (such as different variety of legos, access to expensive art tools/coloring books, musical instruments to play with, etc). When they grow up the average IQ of children from high SES tend to be higher compared to children from low SES

.