Yeah it would be cool to have an underwater case, but would I buy it? Probably not. The bottom line is just to use my phone once a year (or less) for underwater photos when I am on holiday someplace is just not worth.
Its all a question of demand. First take into account that not a lot of people own a Lumia 1020, now from those few people that do have the phone, generously take 5% who would buy the waterproof case, and you are down to pretty low number of people who would buy such a case and thus it probably does not make commercial sense to even invest in R&D for such a case.