On the music side you had Frank Sinatra, then Elvis and then MJ. But it seems now there isn't one person that is at the top. And anyone that gets near the top falls just as quickly.
First, that says more about how little there was in the past, than it says anything about today. The fact is that the world is getting more inter-connected, and options and offerings are more frequent, more plentiful, and more varied. Instead of having to settle, people can now engage in tastes far more specific and unique than ever before, and that works against narrow prevalence.
Second, I think the term "the Great Entertainer" is misleading. It implies that there can only be one of quality, and that quality itself can only be defined by the vast majority of people all agreeing with each other at the same time. It isn't
greatness that these entertainers of old necessarily had, at least not in that context, but rather, as I alluded to before, singular prevalence: They were simply "the most popular at the time" -- that, and that alone, earned them the misleading moniker. There are entertainers today who are every bit as "great" as any of those folks were, even though fewer people appreciate them. The general population moves from favorite to favorite so frequently, now, not because none of them are great, but rather because there are so many of them that are great, and also because the general population has taken a shine to building people up
and tearing them down. This vindictive and malicious nature of the general population is a relatively new phenomenon, becoming a major force only over the last thirty years.
Third -- and I think this is obvious -- there is no advantage (to anyone other than the one who holds the title) to there being a one "most popular" entertainer. It doesn't serve any public good I can think of.