Multicasting seems provide an efficient method of routing networking traffic from a source to multiple end users, especially at this moment in time with teleconferencing, streaming media, online collaboration tools in high use. Looking into it, it appears to be seldom used for such applications, why is this?
Because multicast is one source to many receivers, and thus two way communications (and anything using TCP connections) won't work. That makes it unfit to use for teleconferencing, online collaboration and many more applications.
Streaming media would work, but many people like to be able to pause the stream for example, and that wouldn't be possible with multicast.
To add to that, multicast is quite complex to implement, especially between networks. It is used however, but mostly only within networks. Many consumer networks I know use multicast to provide IPTV with a fallback to regular unicast if functions like time shifting are activated.
Multicast is one of those ideas that sounds nice in principle but doesn't really scale to a network like the internet. It requires routers to keep track of a bunch of extra state, has significant potential for denial of service attacks and also has significant problems from a billing perspective. The result of this is that ISPs generally said no to multicast.
Multicast can be a useful tool within networks you control. It gets used for reimaging PCs in enterprise/academic scenarios and for distributing TV services by IP based "triple play" providers but it's of no use if you want to make a service that operates on the open internet because the providers don't support it.
Multicast was once a popular choice for applications which had the following characteristics: a sequence of events with a similar structure but different values, each new value replacing all earlier values. There are no resources to buffer, or even notify the publisher if an event if was lost, you would just wait for the next event.
Data recorders for physical events and stock market tickers work on this principle. In financial markets most multicast systems now include a parallel request-response system for recovery of lost events.