Here is the anwser for the slow americans.

It is, because when it comes to moral questions, most christians use God as the begining, middle and the end of the entire argument. Therefore, when someone rejects God, not only do they reject your entire justification in all 3 steps, but also they see, that you had to use God to justify these moral opinions, and they just assume, that the oposite must be true.

But, as an atheist in an atheist country, I lived through childhood in a place, where logic chain God -> God -> God was NOT used, and the real moral questions were therefore discussed on their actual value. And guess what? We did find real justifications to hold the same opinions.