There were too many flagerant violations of federal law for the SCOTUS not review the Florida case. Look at 2 USC covering the elections of Senators and Representatives. There is not much there to rule on. 3 USC which covers the Presidential election, offered several rules (like states can not change there rules after the election) which gave solid grounds for an appeal. The Florida case was a slam dunk.
This is an election to the US Senate not the state Senate. The requirements, method of election, and length of term of US Senators is specified in the United States Constitution. Not only is this a federal issue, it is one of the most important issues that can come in front of a federal court. The legislative branch of the federal government is a co-equal branch to the executive and judicial branches. If the state laws and procedures for electing electors to the Electoral College are grounds for filing lawsuits in federal court, surely the election laws and procedures for electing US senators and representatives are also grounds for filing lawsuits in federal court.