As someone who lives in Wisconsin, I can say that it seems to me that Dan Kelly ran a bad campaign. Actually, I don't think he seemed a particularly great candidate to begin with--I think Jennifer Dorow, the other "conservative" judge in the primary who Kelly only narrowly beat, was a better choice in both quality and electability--but independent of that the campaign seemed bad.
When I saw political ads in favor of Prostasiewicz (the liberal candidate who won), they were all about how she would protect abortion rights and Dan Kelly wouldn't. This was actually a major turn off for me, I don't think a judge should be campaigning so directly on issues--but it was still a message and undoubtedly got some people to go for her. Meanwhile, the anti-Kelly ads (not counting the ones that simply attacked him from a pro-choice standpoint) pointed to things like his apparent involvement with the "fake electors" scheme, which is unpopular (Ron Johnson got re-elected as Senator in 2022 despite attempts to tie him to it, but from what I can tell his connection to it is more dubious than that of Kelly's).
So, against this, what was the strategy of Kelly or his allies? Well, I saw some ads attacking Prostasiewicz for being soft on crime in a few cases (they were of the "look at this terrible guy who got a light sentence!" variety which I honestly tend to disregard because I expect such ads are leaving out important information). And... that was it. I actually don't think I saw a single pro-Kelly ad on TV or YouTube; oh, there were anti-Prostasiewicz ads, but not anything that specifically advocated for Kelly. I saw a few banner ads online trying to say that money from out of state far left groups were supporting Prostasiewicz, but again not really anything specifically in favor of Kelly.
If you aren't giving people any actual reason to vote for you, you're not running a very good campaign. Now, I'll be fair and note I live in a fairly liberal area. Maybe they just gave my area up for lost and were concentrating their ads elsewhere in the state. But at least where I was, I never saw a single ad give me an argument to vote for him. So from my vantage point, it seems he just ran a really bad campaign.