Answer and Explanation:
Founders did this because the House of Reps and the Senate have very different responsibilities.
For example, House members need to be at least 25 years of age, while Senators must be at least 30 years of age.
Why?
House members represent just an individual district, while Senators represent their entire state; Senators thus have a bigger burden on their shoulders, and the founders hoped for more mature men and women to take on those responsibilities.
Not only that, but House members only serve for 2 years, while Senators serve for 6 years. Since Senators will be in their position for a longer time, people need to ensure that the person they elect can stand up to their job.
Hope this helps!