Let me answer this question in a few ways.
First, I am not certain if atheists attack Christianity the most. I think you are probably right, but it would be nice to see some statistics.
Second, it makes sense that atheists attack Christianity most, because by and large Christianity is a Western religion (at least of late). More importantly, the West believes in the freedom of expression. So, Christianity which has shaped the West allows for people to criticize it. In other places, like the Middle East, you cannot attack Islam. If you do, it might be the last thing that you do literally.
Third, Christianity makes some bold claim. So, in some ways, it sets itself up for criticism.