Back in the old days when the nation was founded, the founding fathers were a bit afraid of mobs whose will was reflected in the House that has a 2 year term and elected directly from the population.
The Senate was elected by State legislatures (this was changed in the early 20th century, I think) with a 6 year term and supposedly consisted of members of the upper classes of the society and takes a long term of view. I guess they didn't want the mob to descend on the gentlemen nominees.