A few years back, I would have said that they are trying to uphold the image of democracy. “Vote for Harris, a guy you’ve never heard of or this criminal, your choice” isn’t a good look… sad that we ended up here anyway.
Probably also trying to avoid the headache of “what happens when a candidate is sent to prison”. That’s either going to be a lot of work for you or someone you know higher up, who isn’t going to like that paperwork, especially such a high profile case.