Because, to be a pop star you need more than talent. You need the shine, you need the charisma.There still are a lot of talented artists out there, but they stay underground because they are, to be honest, people who aren't born to entertain. The audience want to be entertained. Most people want to be entertained. Talented artists are not the first to come to mind when people want to be entertained.
But of course, if your name is Beyoncé, you can have the ultimate talent, shine, charisma, stage presence, star power and still entertain people. This is why she is the King... or the Queen... or both.