In America, how did being a Farmer go from a profession some of the poorest people had to a job that was one of the most expensive to even start doing?

Technology. Way back when, farmers planted, tilled, and harvested, by hand and with the help of horses, and manually operated tools and machines of the day. As technology increased, making farming more semi-automatic, I mean people still drive trucks, tractors, combines, etc, the tools and machines became MUCH more expensive to buy, operate and maintain. So, farming, became a much more expensive career to get into.