1) Ds would risk losing that seat in 2026
2) He has not been near as an effective attack dog as Walz has proven to be (he came up with the weird framing)
3) He does not have any executive experience which means he did not have the resume VP Harris wanted. While he has been in the Senate the same number of years (4) VP Harris had been when she was chosen as President Biden's VP, unlike VP Harris, he has no executive experience. She was AG of the largest state in the country which means she was executive of one of the largest agencies in the country. That in turn meant she knew how to lead a large organization. Senator Kelly does not have that on his resume. All of the governors did.
VPs who can do the job generally come in two categories - long term Washington politicians (usually Senators, see Biden, Gore, and LBJ) or folks with executive experience