An IRIS-T is for area defense. It’s an easier problem because the target is generally headed right at you. Part of the issue here is that the Navy is largely doing mid-course intercepts which requires a significantly higher performance interceptor in most cases. The AEGIS-BMD mission was designed primarily around fleet defense, not swatting rockets out of the sky which are targeting things much farther away. It can obviously play that role, but most of it requires using much higher performance missiles to do it.