tl;dr: Because there were ongoing investigations (which was true) and it's generally considered bad to release your evidence before trial, or something like that, IANAL.
This will also be Trump's (false) reason for not releasing them.
IMO the most egregious reason is the July 2025 memo from DOJ/FBI saying there was nobody else to investigate, after months of public interest and official statements they were working on it. If they now flop back to claiming they can’t release because of investigations, then that’s unequivocally a false reason.
This will also be Trump's (false) reason for not releasing them.