Only one-third of the Senate is up for re-election during any single election year because the framers believed that

This was a way to make the Senate resistant to public pressure.

Explanation:

The framers of the constitution and pioneers of the American government divided the senate into three so that they can be up for reelection in two, four, and six years intervals. Their intention in allowing senators to remain in the house for slightly longer terms in office was to make the Senate a stabilized body who could take responsibility for their decisions, and who could make decisions that were independent of public opinion and pressure.

The Senate was also formed to provide the needed adjustments to the legislation made by the House of Representatives.