Ask HN: Why doesn't Israel's Iron Dome defense work better?

The rockets that Hamas fires into Israel are usually described as fairly unsophisticated. One often reads about sophisticated guided missile technology being developed. And yet Israel's "Iron Dome" lets enough rockets through to merit killing a few hundred civilians in retaliation every few years. Why is it hard to prevent Hamas's rockets from doing damage in Israeli territory using some sort of shoot-down technology? I don't propose we discuss whether that is a good long-term solution, and certainly not discuss which side is in the right. I'm interested in the technological question of why it is hard to do with a low failure rate.