Do any of the Christians here have an opinion as to why this is the case?
This has been something I've noticed as well.
Christianity is extremely diverse, and many people are not willing to acknowledge that. It is this diversity though that I enjoy, and why I was willing to acknowledge that I am as well a Christian.
It seems to me, at least in recent debates here, that some simply refuse to see that Christians can believe a number of things. They demand that all Christians follow the same basic tenants, and when someone goes outside of that narrow view, they accuse them of not being Christian (Christians do this as well. I have been told by a few Christians that I can't use the title, which I find funny).
For these individuals, I think more of a hatred or intolerance for other beliefs is what is fueling the problem. There are many people of other beliefs that do have misconceptions of Christianity, but are willing to at least listen and learn more about the religion. And that is wonderful. However, the ones who refuse to listen, and assume that they must be right, and try to define Christianity for any other religion, are, in my opinion, only doing so because of hatred and intolerance.
This can be seen with any religion though.