I think it's because they're the only set with a really limited range of countries where they were sold. All the other sets had some global release availability, but I'm pretty sure the seasparkles were only available in North America. Might be wrong on that but I think it's the case.
Add that with the fact they are very pretty, and prices go a bit mad. But I saw a Wavy recently for a really mad price. Don't get caught in a bidding war. Look for a decent BIN or a pony hidden in a lot or wait it out for a bit, for sure.