There are many flaws in all systems, but courts can rule on issues that they can make a difference to does not seem the most egregious.
Courts are not supposed to be the government, so where is the flaw?
Well... upon some more reflection, I concede that there is no flaw. SCOTUS could have provided a ruling, had they chosen to do so. They didn't, and it was probably right choice, even though I find Justice Alvito's dissenting opinion well reasoned and logical: the review would indeed add clarity and be greatly beneficial for future elections - similar issue WILL come up agan.
However, this position ignores that those future benefits probably wouldn't outweigh potential damage that spilling this can of worms could do right now.
As far as I understand, Trump administration shamelessly sabotaged Postal Service, to prevent as many mailed ballots as possible from getting counted in time.
In response, Pennsylvanian court overruled a very specific and unambiguous deadline in federal legislation.
Apparently, SCOTUS does not wish to approve, because it would create a precedent inviting complete anarchy.
On the other hand, it clearly does not wish to add fuel to claims of "election having been stolen/rigged", when Pennsylvania rather prevented the election from being stolen by underhanded executive.
Hence, they refused review. Probably the right choice.