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Scalability issues, no interest from commercial ISPs (I suspect). It was the precursor to modern multicast routing protocols (PIM), but its still a challenge to run large multicast networks today. Unicast (with CDN's) just won because it so much simpler. Multicast today is mostly used by ISPs for their own IPTV services.

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And in large corporate networks.

Indeed, we run multicast over 4 continents. One problem with multicast is the lack of real understanding by network engineers, because it's so rare (certainly anything that involves PIM), few people really get it, fewer tools are available to work out what's going on.

In the broadcast industry, multicast at a local level is very important though - with SDI being replaced by things like 2022-6 and 2110. I believe that's mainly at the IGMP/layer 2 level though, and there aren't many 2110 implementations over routers.

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