Because domestic rabbits are a different subspecies to wild rabbits. They were initially bred from wild rabbits but deviated from the original genotype (the set of genes that determine an animal) and became domesticated so their requirements and behaviour differ slightly. The reason they are different colours is because of genetics which is difficult to explain but i will give it a try.
When wild rabbits started to be bred for food/pets whatever the reason there may have been underlying genes that code for different colours, black for example. Now rabbits aren't able to detect these genes and so chances are they won't breed with an another rabbit with similar genes to produce a black phenotype (visible trait - so black coat colour), or if they did they would die out as they were easily spoted by predators, but people were able to purposely put these animals together and create different colours, sizes, shapes etc.... and so different breeds evolved. This is much the same for the evolution of the domestic dog from the grey wolf and why there are so many breeds.
ETA: And what Timboh said :lol: