In Mueller’s case, it was because the AG Jeff Sessions recused himself. Congressional investigations have much less teeth and are mostly political on nature. There is no power to indict, no ability to convene a grand jury, and no real power to compel truthful testimonies under oath. I am not sure if their is a congressional investigation at this time either since Dems control the Congress, but there is not a DOJ investigation as their should be.
I don’t expect the Dems to play by their own rules or by the rule of law, but this is exactly what should happen - especially given the expectations and events from the Left over the past four years.
The big story here is that we now know without any doubt that Joe Biden, our sitting president, had been enriching himself through his family by using his political position of influence with other countries. It was also with countries with whom he has to now be seen as compromised in his necessary dealings - China and Ukraine (it was Zelensky’s political enemies with whom Hunter Biden was dealing).
Since I doubt your read it in your biased msm sources, you should know that Hunter specifically refers to paying his father a % of the pay off money.