One cause is a decline in intelligence. The Agricultural and Industrial Revolutions of the 18th and 19th centuries and the medical advances in their wake reduced child mortality to such a degree that it switched from being so commonplace that members of the lower class had an effective rate of reproduction near zero to being a rarity.This switch had two consequences. First, instead of a downward class mobility that replaced less intelligent lower classes from above, lower classes began to outbreed the higher. Second, it removed the screen on deleterious genetic mutations, which normally would be removed from the population when their presence in infancy prevented survival. Instead, medical technology allowed these slight defects to remain and become more numerous in each generation, to the point where this load of defective genes now depresses intelligence and retards mental function.
Why is it presumed that the lower classes are less intelligent? I hail from the lower classes myself and have never noticed that the upper classes were any brighter than many of the people I grew up with, many of whom were quite bright.