"the Pennsylvania State Supreme Court ruled that the extended deadline was legal and constitutional under its interpretation of the Pennsylvania state constitution."
Yes. And that was Roberts' reason for allowing Pennsylvania latitude, but not WIsconsin.
The question here is whether or not a state should be able to violate its OWN election laws. Again, the "every vote must count, law be damned" people will take one side, and the "it is essential to uphold the law, regardless" people will take the other.
In Bush v Gore, the Court ordered Florida to stop the recount 7-2, despite it being ordered by the Florida SC, on the grounds that the Equal Protection Clause was being violated by the recount. 7-2 is not a partisan decision, and permitting a violation of the Equal Protection Clause is not a trivial matter.
So, there is precedent for the Court to intervene IF it believes that a State is acting in a way that would disenfranchise the votes or the rights of the citizenry.
What the reasoning will be on the prevailing side when the PA case is again argued is not known, but Justice Alito was pretty clear when he wrote this:
"The Supreme Court of Pennsylvania has issued a decree that squarely alters an important statutory provision enacted by the Pennsylvania Legislature pursuant to its authority under the Constitution of the United States to make rules governing the conduct of elections for federal office."
No liberal democrat (little "l" and little "d", traditional definitions) supports the idea of an unelected Court putting aside the will of the people. Further, the PA Legislature acted on the referred to law (Act 77) in March of 2020, considered extending or changing the deadlines for the Nov 2020 election, and declined to do so.
So, there's where we are. The democratically elected representatives of the people considered changing the statute and decided not to do so. The PA Court, OTOH, has decided to ignore their will.
If this were an international actor, we might rightfully brand them as a "rogue state." :-)
Personally, I am not disturbed by the SCOTUS (figuratively speaking) spanking a State Court and telling them that in the US of A, you have to uphold the laws of the people.
Apparently you disagree. :-)
At any rate, feel free to ignore the broader points I raised, though, since you appear willing to do so.