They sued because it was a state law, a law revisited in the summer of 2020 by the state legislature, who made several accommodations for Covid and purposefully did not change the date ballots could be received.
The US Constitution gives power to the state legislatures only to make state election laws.
In Pennsylvania the State Supreme Court said because of COVID they would force a temporary change in the law the legislative branch recently decided against changing.
They became the law making arm. Trump’s team said it was unconstitutional and I look forward to reading why the court disagreed.
It was not election affecting, but it was a seizure of power by the Judicial branch. Democrats should worry it was not the new 7-3 conservative majority preparing a path to assume more powers theirselves going forward