Because (lack of) authenticity, (lack of) trust, and (lack of) self-confidence/assertiveness are far more powerful factors in determining how persuadable voters will vote than an 'extreme' ideology.
Right, the difference with Oz compared to a generic weak Republican is how poorly he's been doing among Republicans. Nothing about his career to this point suggests that he cares about any of what Republicans care about, and his campaign has barely even tried to show Republicans that he's one of them. The whole thing comes off as entirely cynical, which fuels the attacks against him.
