The correct analysis of why the U.S. Constitution didn't include direct election of senators is best reflected by the option:
It was a compromise made to get the Constitution ratified.
When the Constitution was being drafted, the framers faced significant debates about how representation should be structured. The decision to have state legislatures elect senators was a compromise to balance the interests of large and small states and to ensure that states retained a significant role in the federal government. This arrangement was intended to address concerns that direct election might lead to more populist sentiments that could destabilize governance. This system was later changed by the 17th Amendment, ratified in 1913, which established the direct election of senators.