He is an opportunist.
He entered the race late.
He gets flustered easily on simple questions that he should have been prepared for.
He does not want to debate.
He does not want to sign a no-tax pledge.
He has out-of-state liberal advisors.
He is not a California native.
There is a better qualified alternative (McClintock).
RINOs tend to do the opposite of what they pledge to do in their election campaign.
If the Demos have a dirt bomb on him, they can drop it any time to defuse him, or use it as blackmail.
Here's a bonus reason:
I really don't need anyone to tell me how to vote anyway. I'll vote for the best candidate and if other's don't, that's their problem, not mine.
He's also a dual citizen of Austria. Shouldn't we be calling on him to renounce his Austrian citizenship before governing a U.S. state?