Tails isn't a fighter. Almost his entire movelist has to be shoehorned. Knuckles is. Tails may be an "important character" but he doesn't have enough moves or usefulness to be his own character. He'd fit so much better off as an assist trophy with his mechs.
What was Tails able to do in the games? Everything Sonic can do, and fly. Knuckles had more of his own abilities and could glide. He was his own character. Tails had almost no usefulness unless a second player was controlling him and then he was used to fly Sonic places. That's it. He's never stood out as a character that could make it on his own, so why make him a fighter?
What was Tails able to do in the games? Everything Sonic can do, and fly. Knuckles had more of his own abilities and could glide. He was his own character. Tails had almost no usefulness unless a second player was controlling him and then he was used to fly Sonic places. That's it. He's never stood out as a character that could make it on his own, so why make him a fighter?