I agree with Moonbreeze that they were really badly distributed here. I only ever saw them once in Hamleys after I moved to London the first time and I swear, if I'd known they were never going to be seen again I would have bought more than 2, even though they were a little higher than normal retail there. The Euro exclusives never even made it here I don't think. I found Starswirl in France by accident on a trip to Lille. I never saw Dewdrop Dazzle's original set here either, although I'm told they did come - also badly distributed.
I think knowing that was the case created hype which in turn created the price hike. So it's a combination of Hasbro's distribution vs a sudden surge to want to get them I guess? I'm still hoping to pick up the ones I'm missing at carboot sales as G4 starts to fade out...
It's not just that set though.
As for Holly Dash and the 2nd wave rainbow powers, that one was to do with the dock strikes in the US I think. We definitely got a lot of Holly Dash here but in a very short timeframe. I bought her on discount, again by fluke, around my birthday because I wanted to treat myself - that was the Feb and by April when I became aware that so many people were after her, she was gone and never came back in my area. But I've found her second hand and I expect to find more. In her case it's because she only made it to a small area of the US, Canada and I think briefly the Netherlands? She had a poor distribution too.
Part of it seems to be the pattern of not being able to choose which wave you get in terms of stores. Look at the Hippogriffs. They come up with the same code as wave 1 and 2 baby sea ponies, but they've turned up hardly anywhere as a result. Wave 3 pearlies were a bit like that too for a bit, especially here where, again, they only appeared once. I believe you can still get Coloratura's set in Entertainer here though (unless Christmas cleared them out...) and that of course is wave 1.