It’s obvious why they dismissed all charges of partisan bias.
The IG report says there is no bias there.
It doesn’t actually say that there was no bias.
What it says is that no documentary evidence could be found to demonstrate that bias affected investigatory decisions.
That sentence leaves a great deal of room for a reasonable person to INFER from The details disclosed in the report that bias existed.
Someone parsed this language very closely, and my guess is that it wasn’t the IG.