Why is the left making excuses for Biden?

The left is going after Trump for supposedly threatening to withhold funds from Ukraine unless they did a favor for him. They are saying this is an impeachable offense. Yet Joe Biden…by his own admission…did the same exact thing. Joe Biden is a candidate for POTUS. Yet the left doesn’t want anyone looking at Biden. Kamala Harris says we should leave Biden alone. I have heard that his actions are in the past and don’t matter. Why isn’t there a full scale investigation into this happening?