THe main reason is Biden was a normal President and wouldn't interfere with the DOJ in the course of its normal functions.
And normally the DOJ doesn't release all the details about closed criminal investigations, particularly where there are sensitive details about underage victims.
What people on the right seem to have a hard time understanding is that Joe Biden wouldn't order the DOJ to release embarassing information about political rivals. A decent human being wouldn't do that. The right can't seem to understand that just because Trump would do it, doesn't mean that anyone would do it. Trump may weaponize the Justice Department, but Joe Biden was not the kind of guy to do that.
Also, FWIW, Trump promised to release the file. Biden didn't make a promise. He never mentioned them. He was too busy pulling the country out of a pandemic and recession.