Zelensky is not responsible for what came before him and, if you haven’t noticed, he’s kind of busy right now.
Also, it would be grossly stupid of him to open an investigation into the Biden Administration that is right now his #1 source of support.
Feel free to write to your Senator/Congressman and ask them to investigate Hunter Biden’s laptop. We have all the evidence we need right there.
Zelensky is responsible as it's about a crime syndicate having influence on the Ukrainian government.
As for being busy, he's busy now because he didn't open the investigation when he first got into office.