I spent a decade working on multicast, helped write some of the second generation multicast conferencing tools, and co-authored a number of the standards. Generally, the issue was that multicast requires per group or per-source state in routers, and router memory was a scarce resource. This made it hard to scale globally. It also made forwarding problems hard to diagnose, taking up operators' time. These costs, plus an unclear connection between who benefitted from multicast and who paid for it made it difficult for core ISPs to justify. And that meant that commericial content or application providers needed to implement a plan B in case multicast wasn't available, and if that worked well enough, they didn't bother with also doing multicast.

Still, RTP and SIP came directly out of the MBone, and they bootstrapped the whole Internet multimedia phenomena we're all enjoying today while working from home.