Bush stopped effectively defending himself against the left wing democrats, and therein lies his fault for his unpopularity. That’s without even addressing the substantive things for which he might deserve unpopularity.
It is not only Bush's resonsibility to defend himself. It's typical for people nowadays to look only to the top as though we have a king.
One man cannot defend the entire GOP movement and policies. It's up to the whole GOP, including the grassroots who often like to spend all their time stabbing their own in the back.