The Federal court got involved because the State court was in effect changing the rules of the election after it was held.
I tried researching what I know to be an historical fact but couldn't find it.....
The US Supreme Court ruled in the middle to late 1800's that in a Federal Election, no state may change the election laws of its state. Any change must apply to the next election. I believe this was the basis for the vacating of the Florida Court ruling, although not identified.
The 1st part was based on the separation of powers decision by Chief Justice John Marshall. There was a subsequent decision that ruled that since each branch was equal, no decision by the court could be agreed to except following an election decision...
Thus, the Florida Supreme Court could not impose itself on a Federal Election except for guidance in the next Federal Election. However, the Constitition leaves it to the State Congresses to determine the terms/means of election. Therefore, and in any case, the Florida Court was wrong!....