Very simple.
Traditional fly reels have to be filled up with lots of backing, far more than is necessary. Not only that, the small diameter of traditional reel cause horrid kinks in fly lines. Not only that the small diameter means that the line retrieve rate is slow.
Large arbour reels are marvellous and worth every penny. I own two Dragonfly Large Arbours which are not expensive and they are British.
Have a look at them.