I did wonder that the first time I saw one of these programmes advertised, but I concluded that, as others have said, that due to the NHS and advanced medical techniques, combined with a lack of cultural superstition, inbreeding and (generally) excellent pre/post natal care, the type of conditions that are featured on these programmes are identified, diagnosed and treated at a much earlier stage in the condition, and probably less prevalent in the first place.
I don't know to what extent 'better' diets and clean water makes a difference. I would also argue that not all people in this country do have 'good' diets. They might eat more food, but that doesn't mean their diet is any better nutritionally.
Alice I don't really get what you're saying. Are you actually suggesting that a belief in gods such as ganesha means people are more likely to be born with more arms (and a trunk?!). I would suggest that reproducing with close relatives has more to do with it than religious beliefs!!! 