AFAIK, turning on multicast would create many additional RIB/FIB entries in routers, possibly running routers out of RAM and requiring upgrades.
Also, because multicast duplicates packets, it could increase bandwidth usage in ways that are not easy to bill for, potentially increasing costs without increasing revenue. I don't think this is a real problem, but some people have cited it.
IP addresses (which we're running out of) address hosts. There are hundreds of millions of active full-time hosts.
Multicast addresses address content. How much more content is there than hosts? Do you really think it's feasible to give an IP address to every popular piece of content? YouTube allegedly gets almost 100,000 new videos per day.
The fact is, multicast hasn't been a success because it's simply not a good fit for the routing layer of the Internet. But edge overlay networks can build arbitrarily complex and interesting multicast service models on top of the Unicast Internet, and those overlays have been huge successes.
No, SSM reduces an explosively intractable routing problem to merely an intractable routing problem, by guaranteeing that video streams will only have a single sender (which is not even a win when you're competing with edge overlays).
You still need to have every upstream router maintain awareness of every piece of content being viewed by every downstream user.
