Have you read it? Because he didn't choose Johnny because he was a fire wielder. He chose Johnny because Reed had just switched his and Sue's powers. He wanted the invisible person, he detected the invisible energy signature in Johnny, and zapped him, otherwise he would have done it to Sue.
The Fallen One, Silver Surfer, Air-Walker, Firelord, Terrax, Nova, Morg, Destroyer, Red Shift Invisible Man (just in normal continuity, there have been some other reality ones we've glimpsed at, but to my knowledge none of them were fiery).
So there have only been two fire ones, and it can be argued that there have been two air ones (invisible man, and air-walker). He's also had two guys with axes. Once we get to an air controling fire headed axe wielder, then I'll agree something's up.