“Why would the Supreme Court take this case at all instead of leaving the blame for the 2020 election fiasco right where it belongs — with the state legislatures?”
Because the Supreme Court is there for suits that rise above the level of the State Supreme Courts.
In PA the election laws are written into the Constitution. Election laws were modified as a result of a lawsuit (NOT the State Legislature) and the PA SC ruled to move ahead with those changes. That is blatantly unconstitutional from what is written in the PA Constitution. The SCOTUS is the place to take that suit next according to the US Constitution.
They punted....unconstitutionally so.
If the matter relates to the Pennsylvania Constitution, then there is no reason for a Federal court to be involved.
The PA legislature didn't even take any steps to stop that fiasco from unfolding.