Elton John is a gay icon. He was openly gay when it wasn't barely tolerated in polite society as it is now. Having a straight actor play him might seem troubling to gay advocates and gay rights people as it erases representation in a way. It makes it all easier for straight people to shrug over the Elton John movie as we know the actor is straight. It's rather like replacing an Asian character with a white one-- as was done in Dr. Strange, Ghost in the Shell.
As far back as 1915 — when Mary Pickford starred in "Madame Butterfly" — producers and directors have had no problem casting white actors and actresses as people of color. John Wayne, Katharine Hepburn, Anthony Quinn, Boris Karloff, Fred Astaire, Marlon Brando, Alec Guinness, Mickey Rooney, Joel Grey, Linda Hunt, Max von Sydow, Peter Sellers, David Carradine, Mike Myers — and many, many others — have played characters of Asian descent.
Representation matters. It might seem ludicrous to those who are overly represented by Hollywood. But it matters. In this case, with the backlash against the LGTBQ community happening, with rollbacks of rights very well leading to rescinding legal same-sex marriage, a gay actor playing one of the most openly gay performers of all time would be an actual edgy thing to do. Casting a straight man is utterly safe, boring and about blanding down this topic altogether.