You thinkin' maybe the Founding Fathers weren't as smart as we'd like to think?

[Benton D Struckcheon]

22. That explains their election by the state legislature in the original,

but as Madison makes clear in Federalist 62, there were other reasons to have a second body:

First. It is a misfortune incident to republican government, though in a less degree than to other governments, that those who administer it may forget their obligations to their constituents, and prove unfaithful to their important trust. In this point of view, a senate, as a second branch of the legislative assembly, distinct from, and dividing the power with, a first, must be in all cases a salutary check on the government. It doubles the security to the people, by requiring the concurrence of two distinct bodies in schemes of usurpation or perfidy, where the ambition or corruption of one would otherwise be sufficient. This is a precaution founded on such clear principles, and now so well understood in the United States, that it would be more than superfluous to enlarge on it. I will barely remark, that as the improbability of sinister combinations will be in proportion to the dissimilarity in the genius of the two bodies, it must be politic to distinguish them from each other by every circumstance which will consist with a due harmony in all proper measures, and with the genuine principles of republican government.

Second. The necessity of a senate is not less indicated by the propensity of all single and numerous assemblies to yield to the impulse of sudden and violent passions, and to be seduced by factious leaders into intemperate and pernicious resolutions. Examples on this subject might be cited without number; and from proceedings within the United States, as well as from the history of other nations. But a position that will not be contradicted, need not be proved. All that need be remarked is, that a body which is to correct this infirmity ought itself to be free from it, and consequently ought to be less numerous. It ought, moreover, to possess great firmness, and consequently ought to hold its authority by a tenure of considerable duration.

In other words, because a single body subject to election once every two years could wind up with a bunch of total demagogues, and some check would be needed on their propensity to do stupid things.