I am not a lawyer, but it seems to me that we have the presumption of innocence for defendants in our criminal courts because what is at stake for them is their freedom and sometimes their lives.
Those are high stakes, and thus the burden of proof is on the state.
The stakes are different when we are talking about whether a person merits a lifetime appointment to the highest court in the country. Therefore, the balance and burden of proof ought to be different as well.
This question seems neither difficult nor controversial to me.