Final answer:
Some states did not send two senators to Congress because their selection was based on appointment or selection by state legislatures. The Seventeenth Amendment passed in 1913, changed the selection of senators to direct election by the people.
Step-by-step explanation:
Prior to the enactment of the Seventeenth Amendment, some states did not send two senators to Congress because the selection of senators was based on appointment or selection by state legislatures. Each state had the power to determine the method by which senators were chosen. However, due to a series of scandalous elections in the late 1800s and early 1900s, citizens became resentful of senators who were perceived as being selected based on personal connections rather than qualifications. As a result, the Seventeenth Amendment was passed in 1913, which changed the selection of senators to direct election by the people.