In the late 1800s, what factor most contributed to the successful implementation of the practices of vertical and horizontal integration?

Lack of government regulation of business practices.

Horizontal integration is the practice of buying smaller businesses creating competition so a company can have a monopoly over the sale of an item. Vertical integration is the practice of buying companies that supply the process of of a manufactured item from raw materials to transportation.

Corporate tycoons of the Gilded Age were able to use these economic practices because there were no laws or regulations to prevent them from doing so. John Rockefeller was an expert at horizontal integration. He bought oil industries out so he could be the sole provider of oil in America. Carnegie was an expert in vertical integration. He bought iron mines, creating steel mills, and bought rail lines to transport his goods. These practices made tycoons wildly wealthy which allowed them to continue buying and investing more to become more wealthy and powerful.

the answer is mass marketing.