Why aren't block solutions sent to a multicast address? In computer networking, multicast (one-to-many or many-to-many distribution) is group communication where information is addressed to a group of destination computers simultaneously.

I think the Satoshi client uses Universal Plug-n-Play (upnp) to find other Bitcoin nodes.  I came to this conclusion after a cursory glance through the code at occurrences of "multicast," IIRC.  I was looking because I thought that multicast would be a great way to broadcast block solutions.
Multicast just doesn't work very well over the Internet. Relaying just works better. One big problem is that multicast provides no way to police emitters of junk while relaying allows each relayer to verify information prior to relaying it, preventing an attacker from getting amplification.