Because Kaus isn't as far left as Boxer -- and lacks a political machine and fundraising -- he'd probably have a tough row to hoe in the primary. The California Democrats like their candidates liberal.
He'd be a much better senator than Boxer, it goes without saying, and having an occupant of the Boxer seat being informed by something besides the latest lefty preoccupations and talking points would be a pleasant change. Then again, when it comes to being better than Boxer, it's hard to find a candidate who wouldn't qualify. After all, you can't fall off the floor.