Maybe because nearly 50% of voting population actually preferred Bush/Trump over the alternative? Especially when federal government has vastly increased its power (defense, education, environment, taxes, social policies) and when president has the rare power of nominating supreme court justices who can then stay forever deciding crucial matters (corporations, abortion, gay marriages, affirmative action...). In such cases, its better to vote for a bad candidate on "my side" rather than a better candidate from "other side".
In fewer words - hyper-polarization. Once you win republican or democratic primary, you are guaranteed to win 45% of the vote.