It's fairly simple really; this was an argument that needed to happen before the election, before millions of people voted. The rules should have been corrected then. After the election... it's too late.

Maybe an argument could have been presented for re-running the election in that state, with the rules corrected. That would have ensured that the true victor took Pennsylvania. But that argument wasn't made. Instead, the "losing" side sought to have a bunch of ballots discarded, handing themselves the victory. And that argument was always going to be a non-starter.