It is worth remembering that Republicans went to the United States Supreme Court on an emergency basis in both Pennsylvania and North Carolina raising the same theory: that the extension of the deadline for the receipt of absentee ballots was unconstitutional under the “independent state legislature” doctrine because it usurped the power of the state legislature. In PA, the state Supreme Court extended the time for the receipt of absentee ballots to Nov. 6, relying on the state Constitution. In NC, the state election board negotiated a settlement with those suing over voting rules to extend the deadline for the receipt of absentee ballots to Nov. 12, relying on power delegated to it by the legislature to do such things.
Although Republicans sought emergency relief (under a controversial theory) to stop the counting of the late arriving ballots in both states, they’ve dropped the claim in North Carolina but continue to pursue it after the election in Pennsylvania.
It is no mystery as to why: Republicans are ahead in North Carolina, and they want to keep counting votes to pad the lead of Trump and Senate candidate Tillis. But they are behind in PA and are seeking to stop the counting of those late arriving votes. (As Rick P. noted, the number of ballots at stake in PA is 10,000 not nearly enough to affect the outcome in the state).
Of course if the theory that Republicans were advancing is correct, it should apply equally to both states. This is true of lots of other theories too, like the crazy one advanced in the new federal case in PA that allowing people to vote by mail or in person is unconstitutional because different procedures apply to voting in these different ways.