I'm not really sure of the reasons why but it is true, in the course of my work as a pest controller I had capture and re-home 4 white feral cats, 3 of which ere deaf, its a genetic thing i think.
Pure white cats and also dogs are prone to deafness due to a genetic link between coat colour and the hearing deficiency. This is more true if the animals have blue eyes as well.
I dont know about deafness but they are prone to sunburn and cancer - we didnt know this until it was too late - our cat had to get his ear flaps removed because they were being 'eaten' by skin cancer - he was 15 at the time and lived until he was 22.
Our white cat was the same, he got skin cancer on his ears, he made it to 15 odd though but we had to have him put down out of kindness when his suffering got to much, he was telling us in his own way it was his time to go.