IMHO, the scale is too large for a cover up like that. There's too many individuals, companies (many of these email addresses are gmail, yahoo... private email servers) and foreign governments involved.

Many of these emails are in an evidence room already from the 2008 (edit: I guess I'm assuming here for 2008) and 2019 court cases. If you mess with them too much, just for example, some random federal or state level DOJ employee could leak stuff, personal risk be damned. Or, another example, some foreign government might decide causing societal chaos in the USA is worth more than protecting Lord Toucheskids and release what they have.

The only reason they're releasing anything at all is because they have to maintain the illusion of a working DOJ and congress. Too much of the American public is truly mad about this stuff to just ignore an act of congress about it. They released the files in the most optimum way for their goals IMHO: literally everything, an unorganized firehose full of duplicates, random redactions, and a few FBI tip-line interviews that are provably untrue, along with the true ones, and the indeterminate. It'll take forever to sort through it all, and Trump/others implicated are banking on it.