This is an easy one. There is NO Constitutional right for the populace to choose the electors. That's a matter for the states. The California legislature, for example, could decide to allow the Governor to appoint the electors, or for the state senate to vote on who the electors from that state will be. So there's no US Constitutional enfranchisement to begin with, and thus no disenfranchisment.
"Each State shall appoint, in such Manner as the Legislature thereof may direct, a Number of Electors"
Correct but you miss the point. It has nothing to do with HOW electors are chosen, but NOT representing the people OR the state by being withheld is a violation of civil rights.