Because most voters are believers and atheists are non-believers. This suggests we have different values. According to the Demographics of atheism wiki:
A study on personality and religiosity found that members of secular organizations (like the international Center for Inquiry) have similar personality profiles to members of religious groups. This study found that members of secular organizations are very likely to label themselves primarily as "atheists", but also very likely to consider themselves humanists. It was also found that secular group members show no significant differences in their negative or positive affect. The surveyed individuals also had similar profiles for conscientiousness (discipline or impulse control, and acting on values like "pursuit of truth"). Secular group members tended to be less agreeable (e.g. more likely to hold unpopular, socially challenging views), as well as more open minded (e.g. more likely to consider new ideas) than members of religious groups. Luke Galen, a personality researcher, writes "Many previously reported characteristics associated with religiosity are a function not of belief itself, but of strong convictions and group identification."
Virtually everybody shares certain core moral values but other moral values are more open to interpretation. Sex is a notable area of disagreement. It seems the God of Abraham is very concerned about our sex lives. So dutiful believers take their lead from Him. There's plenty of variety in attitudes toward sex between believers and unbelievers, of course, but a significant number of believers, especially among fundamentalists, judge and condemn homosexuality, premarital sex, casual sex, etc. Some even insist sex should be for procreation only.
Politically, believers are associated with conservative values and non-believers are associated with liberal or libertarian values; though, as with sex, there's plenty of overlap here too. Religions are bastions of traditions and beliefs resistant to change. The faithful practice of religion tends to lead to conservative values. Atheism, on the other hand, finds little virtue in religious tradition and beliefs. Instead of claiming the possession of truth, atheists are more likely to challenge truth claims in an effort to advance human understanding and improve society accordingly. This 'liberal impulse' has led to radical changes in human rights, women's rights, minorities rights, sexual liberation and other protections . . . changes that conservative believers have fought tooth and nail. Atheists, as a maligned minority, are usually sympathetic to other minorities and, thus, to liberalism.
As freethinkers, atheists will probably never have a political platform they can get behind in any united way. Atheists come from all walks of life and political leanings. Even if just the liberal atheists were to organize, they probably would not agree on their approaches to most issues and policies. That's the consequence of fiercely independent thinking where just one thing — lack of belief in God — is shared by all.
But this individuality and diversity is an asset, not a liability. Since we don't have a shared ideology that others must accept, we can address issues and policies based on merit without the encumbrance of ideological doctrines and dogmas.
A healthy democracy must protect minority interests against entitlement of the majority. There should be no unfair advantages or privileges. Without the baggage of a religious agenda, atheists are driven by a humanitarian interest in improving the human condition. I think that's the best approach to ensuring a society without unfair advantages or privileges: as well as the best approach to gaining the trust of the voters.