Mixture of everything really, the big one is cost, you can pick up 195/50's from £30 for a Toyo T1R where the 55 is closer to £55, then there's the improved acceleration which considering that on the road top speed is irrelevant then it's a bonus and then the looks as the 55 makes the 15" rim look small. 205s would offer more footprint but at the risk of it catching due to lower offsets/wider wheels