A democracy cannot exist as a permanent form of government when the voters discover that they can vote themselves largesse from the public treasury.
When the the majority of people are employed by the government or receive financial benefits from the public treasury the government will become an oligarchy controlled by an oligarchy of international bankers and Wall Street Corporations.
A direct election senators allows the people can elect their own senators creates a potential for a tyranny of the majority where the ignorant and uninformed masses can violate the rights of a minority.
When the financial elite choose the candidates and we are allowed to vote for their puppets, it really doesn't matter who we choose, we lose.
In an indirect election, the 7,338 representatives of the people in the legislatures of the states chose the senators to represent their state. The people at large participate in the election of their representatives in the House and the representatives in their state legislature choose the senators.
If you were planning to take a trip to the moon would you rather have the people at large choose the crew or would you rather have a panel of astronauts make the decision?It is presumed that the state legislatures are better educated and well informed on the issues and are better able than the people to choose the best candidate to represent the best interests of the state.
In a direct election the people at large make important political decisions, while in an indirect election, it is the best educated and informed representatives of the people who make the decisions.