American political parties tend to promote fairly similar policies because

American political parties tend to promote fairly similar policies because they need to appeal to the same general groups of voters.
Most voters want to hear the same thing - they want equality, better wages, and overall, better living conditions. Thus, all of the political groups tend to promise them the same things because all of them know what their voters want.

The changes to agriculture in the united states since 1900 is that increased mechanization has increased food production, thereby increasing the need for labor. there has been an increased need for higher speeds of food production due the fact that the population in the united states continues to rise.