Because Prawn Cocktail is not a big flavour in Europe or the states and is a british thing really. Pringles are not English in origin so they're not that bothered about thIS flavour. Wish they did though!
Not being an English flavour doesn't necessarily mean that's why they don't make it. There are a lot of flavours here that you can't find in the states.
kelbillug - you are so right about that - I emailed them to ask why you guys can have the Smoky Bacon Flavor and so many others there and we can't here in the USA - they did answer - but it did absolutely no good!!! I have to admit I am a little jealous!!!