Because the original Spitfire already outperformed it. There were more Hurricanes than Spitfires at the BoB, but, even then the Spitfires outflew the Hurricanes.
Why try to make something worse better when you can make a "better" better?
Well, from what ive read, the Hurricane performed better against the Luftwaffe fighters at the time than the Spitfire did, and i believe i read somewheres it achieved more kills during the battle than the spits.
so, you have the spit who performed better than the hurricane, but the hurricane performed the task set to both of them better than the spit.
idk, i just think there was alot of un-tapped potential in the hurricane. Would of loved to see a 4-6 .50 cal version.