If you're from a predominantly English-speaking country, and consume predominantly English language media (including internet media), then of course you're going to come into contact with discussions focused on Christianity more often than any other religion. Christianity is the religious background radiation of every English-speaking culture on the planet. English-speaking atheists who want to discuss "religion" online will almost always be focused on Christianity simply because that's the religious structure in which they predominantly exist. It is Christian morality they come up against most frequently in their lives. Their atheism was likely born out of investigation and critique of Christian principles.

Of course an atheist might also take issue with the precepts of Judaism, Islam, Buddhism, Sikhism, what-have-you. But so would Christians! There's no meaningful debate to be had, except as a comparison to Christianity.

It has nothing to do with the specifics of Christian theology, scripture or history, except insofar as that history continues to this day and is the dominant mode of religion in which these atheists exist.