Young Earth Creationism pseudoscience is one reason, but there are tons of others. Ham, I think, is mostly right: Either (1) it's all true, in which case you have all the problems of fundamentalism and have to disbelieve science. Or (2) or some subset of the factual claims it makes are true - the Bible as miraculously inspired and truthful, YEC, the very existence and nature of the supernatural, Old Testament miracles, New Testament miracles, eschatology, and so on - but as Ham eloquently points out, once you abandon one of the claims that the supposedly truthful book makes, you call them all into question. Or (3) none of it is true, it's all fiction, in which case you can use whatever you want as a moral framework, but it has no special value. In that case, it's merely a socially evolved construct like every other superstition throughout the history of our species.