Why has he not?
Already explained many times. Mueller is using Koskinen to get at Trump’s tax returns. A creep like Mueller can take tax data and twist it whichever he wants even though the tax returns have been audited and have passed examination.
Mueller is the real swamp creature to take down, not Lerner, not Koskinen.
So the idea is to get Lerner’s testimony on Koskinen’s misdeeds, then let Koskinen know he is owned. That neutralizes him.
Result: Mueller is denied opportunity to make trouble via Koskinen.
As soon as Mueller folds or is dismissed, Koskinen will be resigning quickly. His utility is worth more as a buffer to Mueller.
Lerner’s testimony against Koskinen is worth more that hanging her in the public square.
Trump is a student of General MacArthur. MacArthur adopted a strategy of bypassing the largest installations of Japanese military in favor of going behind them and cutting off their supply lines.
Trump is doing the same with Mueller. Lerner, Koskinen are pawns.