Reading to Gatwick

Please can you tell me why the Reading to Gatwick service is only ever three carriages long, and dirty old First Capital Connect carriages. It’s always packed, and uncomfortable. It’s such a busy service, why is this ? I would use another train service if I could but you seem to have the monopoly on this service. 

Why cant you make it at least a few carriages longer, and why are you using old First Capital Connect stock which is years old ?