Pre-17, Senators were conceived as ambassadors of the States to the FedGov. Having the FedGov tell the States how to select their Senators is like the UN telling the various nations how they must select their ambassadors.
Additionally, statewide popular vote for Senators just hands power over to the populous cities. If we must vote directly for Senators it should be by some sort of state level electoral college that preserves the voices of the countryside against the onslaught of the peutocracy(*).
(*)Peutocracy - rule by purile effite urban twits.