I used to think the same way as you, but I've found that many, many conservatives want only Christians elected to office.
I would much prefer a non-Christian who adhered to the principles of limited government and individual rights to a Christian who was a big government supporter. Aside from the issues of laws regulating morals, many evangelical Christians of past generations were advocates of increased Federal power and centralized economic control. Better William Howard Taft, a Unitarian and strong skeptic of government intervention, than William Jennings Bryan, an evangelical Presbyterian who was socialistic in many of his economic views.
That being said, it must be admitted that in our present day most nonbelievers in public life are liberal, as well as most who belong to liberal Protestant denominations or are Jewish. Even many Catholic politicians and clergy are pro-big government, with exceptions made on the so-called social issues.