as said before VHD wasn't around then & there are new strains of mixi about now I think after they were release in Australia about 3- 5 years ago & have spread. This means that wild buns who had developed immunity are now catching it again so pet buns are more likely to be catching it too. As said before it is also warmer so you get more biting insects and for longer periods during the year....all increasing the chances of mixi spread.
Also 20 or more years ago, many of the rabbits around were still relatively healthy from our history of meat production. Now breeding is much more about looks and less about health so I don't think that domestic rabbits have the same constitution as they had then.
Thirdly as also said, my mother in law can remember having rabbits and they were fed from the hedgrow, with some extra veg thrown in. They weren't packed full of processed food that they get now. This can lead to obesity problems and slow digestive tract as they don't get the fibre they need.
Forthly, twenty years ago there were no computer games and much less other distractions and parents were a lot tougher on their kids so if they had a pet it probably got more attention, love and was looked after properly...rather than left to languish in a cage...which usually leads to one sick bunny. I know that if I hadn't cleaned out my gerbils when I was supposed to I would have been put over my parents knee and my gerbils were better off for it.
Finally I think, 20 years ago people were much more likely to right off a rabbit.....they wouldn't investigate why it had died or if they could have done something to stop it. There was much more of an "oh well we can always get another one" attitude. I personally would prefer us to be more like many of the people on this forum are which is to find out whats wrong, learn and prevent it happening again.
ps....I too can't read NHS direct without thinking I'm going down with the plauge.