Why different terms of congress?

The reason is that House is the lower body and will always have a movement of people in and out. The Senate is the upper body and would have people who had the experience and knowledge to temper the lower body. This system worked fairly well until there began professional politicians. Congress felt that a man should stay a short time in public life and then return home after serving the common good. When the country was 90% farms it made sense. Today it doesn't so much.