Roy's Flare Blade is suicide if you charge it up fully while you're really damaged and his Counter may be more damaging but less likely to hit the enemy because it's in effect shorter. Besides, Marth's Dolphin Slash can send enemies flying much farther than Roy's Blazer. And Marth's attacks are the most strongest when the tip hits the enemy which takes full advantage of his long sword range. Having Roy's sweet spot in the center is pointless because using his long sword range to hit your opponent from afar won't deliver the most damage. And Marth looks way cooler and has a way better taunt. And Ken (the world's former Super Smash Bros. Melee champion. Even if he is the former champion the current champion is also a Marth user) has slaughtered every Roy user he faced and has never been defeated as Marth when he uses Roy. Even though Roy's attacks are fiery they're slower and faster opponents could easily interrupt Roy's moves with their own attacks. Marth on the other hand can strikes his oppponents way quicker and can stop opponents from using their attacks. It's better to have strong attacks that will hit the enemy than strong fiery attacks that keep getting stopped by the enemy.