A plausible reason is that it would likely impact their market value as shareholder confidence in those banks is affected by the potential for large fines in the near-future. Since the banks haven't actually been found guilty of anything (yet), they're simply being investigated, it would be damaging to release their names.

The "economy will be hurt" if bankers go to jail is getting a little old, isn't it?

It's also sounds like exactly the kind of thing bankers would like the public to believe and would try to influence media narrative to repeat it.

My point is simply that they haven't been found guilty of anything yet, just charged. If they're subsequently cleared of any wrongdoing, but their market value is tanked by the accusation, they're effectively being punished for nothing.

Innocent until proven guilty and all that.

Here’s one, possible, good reason: the EU has a sort of “whistleblower” defense, where the first company to cooperate in a cartel investigation gets off free. Assuming that bad PR is a significant part of punishment in these cases, it would make sense to delay announcing the culprits here to give one of them time to escape being named.

They still get named in the end but they avoid some of the fine.

As long as they are not found guilty naming them would equal to putting up the pillory. Always a bad idea.