For one, if a legislature had split party control, it would often deadlock on who to appoint and states could go years without Senate representation.
Even the conservative Heritage Foundation has debunked this idea, pointing out that state pols have HUGE incentive to increase federal spending. heritage.org/the-constituti…
By my estimate, if every legislature elected a senator of their own party, the GOP Senate majority would currently be between 60 to 62.