I think nostalgia and rose-tinted glasses for the past make a big part of it with Queen. Before live aid they weren’t doing well as a band at all. Despite whatever the re-writing if history in the movie tells you hmm
The "music business" in-joke when they first signed to EMI was "Whats the difference between EMI and The Titanic?" - " At least the Titanic had a good band".
I think the thing that strikes me about Freddie Mercury at the time was that, yes, he was a very good frontman, but so were many of his contemporaries, such as Robert Plant (Led Zeppelin), Mick Jagger, Jim Morrison , and even Ozzy Osbourne! Peter Gabriel also springs to mind with his early performances with Genesis (although I quite disliked their genre of music), and also in his early career with the Faces, Rod Stewart.
All these artists (and many of the ones already mentioned in the thread) were truly "Front Men" in the sense that they really did engage with the audience, and make them feel included in the performance, as opposed to "just" being the lead singer.
Mentions should also go to the pioneers like James Brown, Wilson Pickett, and previous to that the old Blues "Shouters" (Wynonie Harris, Big Joe Turner), because Freddie Mercury was not just "invented" in the 70's, and didn't come to fruition in a vacuum, but was the pinnacle of a huge pyramid of previous talent that had influenced him and pioneered his style.
And therefore that is perhaps why there are not so many around in the present music business, because many of today's performers are manufactured and produced to reach a very specific targeted audience, and to a certain extent, their talent (or lack of it) is not really a concern, as it is mostly a "here today, gone tomorrow" attitude, as modern technology changes.