Because he can and it's none of your business.
If Giuliani wants to go to Ukraine and investigate something on behalf of a Client. He can! The people he's going to talk too don't have to talk back. But they are! Hmmmmm!
This whole impeachment bullshit is the Democrats trying to prevent people from looking into Biden and his son's corrupt dealings in Ukraine.
There is no evidence of corrupt dealings. Your saying so is not proof.
That's certainly true enough. But Rudy Giuliani is an expert at rooting out evidence. Giuliani took down the 5 Families in New York, he will certainly be able to take down the Biden Family.
The problem right now is that Biden has corrupt politicians in his pocket. Look how many people voted this morning to absolve the Biden Family's documented involvement in the Prostitution and Cocaine rackets, by voting to impeach Trump for even looking into it?