Why did a lift kit change my towing capacity?

I put a 9" lift kit with 37" tires on my truck and now im told that i can only tow 6,000 lbs instead 6,500 lbs. Why is that and how can I raise my GVWR towing again? I always thought that with the bigger tires, I could tow heavier trailers.Why did a lift kit change my towing capacity?
its the tires and lift that make it go down but you could get a programmer to change the ratio settings and be able to pull a little more than 6500 if ur lucky wouldnt count on it though but will notice a difference in power.Why did a lift kit change my towing capacity?
They maybe talking about the new angle of the drive shaft,

any more weight mite shove the rear shaft into the transfer

case or transmission.Why did a lift kit change my towing capacity?
someone told you wrong you get more towing capacity because of the gear ratio ( and how are you gonna get a trailer on a 9" lift you have to lower your ball