I honestly believe discovery would have revealed much.
But ultimately, the SCOTUS is the court of primary jurisdiction in disputes between the States, which ultimately that was. Some states did not follow the Constitutional requirement for their Legislatures to set the election rules, and instead changed them willy-nilly with edicts from appointed or minor officials. Those rule changes made fraud not just possible, but probable (at the least), materially affecting the outcome of a national election.
Just how much evidence did the court review?
Here's a list:
Doubtful. Three years of constant agonizing by people like Trump haven't produced much evidence.
The primary evidence adduced is that there is no really good way to audit a U.S. election; but that is not, without more, clear and definitive proof that massive, election-changing, fraud happened, it is merely evidence that fraud could have taken place without the ability to catch it forensically.
That is not the sort of minefield the Court is going to wade into. Any ruling other than the ruling the Court made would have damaged the Court irreparably.
For better or for worse, the case would have come down to inferences drawn from circumstantial evidence. That would never have satisfied anyone on the losing side of any substantive decision.
What really sucks, however, is that, with the exception of a few intrepid souls like DeSantis in Florida, nobody is taking significant steps to make U.S. elections more fraud-proof by making them more auditable, or requiring more proof of eligibility to vote.
Particularly egregious is Mr. Trump's consummate failure to say anything about the alleged vote fraud other than "I wuz robbed". He hadn't done a g*d-damned thing to put his considerable influence behind a national movement to make U.S. elections as secure or auditable as, say, elections in Mexico, even.
The failure is, at bottom, a political failure, not a judicial failure, and we have been abetting that failure by refusing to galvanize our anger into practical, functional responses.