She didn't have four years to do it, she wasn't even President.
And at that point the wheels of justice were going, just... slowly. There was no reason for Biden to speed up the process either. It was assumed that there would, ultimately, be trials and convictions.
Having the name that appears most, save for Epstein himself, in the Trump-Epstein files become President, coupled with the people he put in charge, including Pam Bondi who was AG during a suspicious period in Epstein's life, and given Epstein died under Trump's watch, made the issue far more uncertain and made it clear it wasn't practical to just assume the courts would do their job.