Why then were Texas, Virginia and Mississippi not able to participate in the 1868 election? Was the readmission of the congressional delegations a prerequisite for their electoral votes to count?
I'm not sure. If I were to hazard a guess I'd say it was dependent on where in the Reconstruction process a state was. At the time of the 1868 election those three were the only former Confederate states that had not yet ratified the 14th Amendment and guaranteed suffrage for their black population.