I suspect that a lot of the problem is that the Republican party is currently pushing for theocracy, under the control of the most extreme, fundamentalist elements within American Christianity. If a given conservative atheist is a proponent of the Republican party ... then yeah, we have an issue, and she won't get invited to many atheist conferences.
Also, on the subject of this group, I suspect that most of those speakers will be very anti-feminist. I don't care if misogyny is coming out of the mouth of a man or woman. Fox News and other conservative outlets pull that kind of shit all the time, holding up misogynistic women as representative of true feminism™.
I do not think it has much to do with the Republican Party itself. Many atheist organizations want atheists that fit into a neat little box that fits their political expectations. Conservative does not equal Republican. A few of the women on the list do not associate themselves with the GOP.
I am somewhat curious, though. Which flavor of conservatism are we speaking of, here? The economic, "Let the free market sort everything out," variety or the anti-feminist, anti-gay rights, anti-minority variety? Or both?
That might help clarify the issue.
Every conservative is different. You have to read about them individually.