@Ass_Slapper Mostly because it's old. Most people don't bother with the classics unless they're already really heavily recommended. It doesn't help that the closest thing this has to an anime adaptation is two OVAs.
@ Mr_Stizblee - sadly so. What's worst is that, this story is so great and unique that is a real shame it did not got animated. The 2 OVAs were really great too, the animation was great. I am just sad it did not got more than that.