Why is it so hard to memorize certain things?

But it's easy to memorize other things. Why is that? What I mean is that I can memorize directions to a location, near or far by hearing it once. Maybe I need to write it down, but I hardly refer to the written directions. But yet when it comes to memorizing Bible verses, very important in the Christian life, I can't. Well, I can, it just takes me a LONG TIME. What is that?