I think it's mostly because A) she's pretty popular, and B) packs themselves, for some at least, are hard to come by. All of the stores in my local area that sell them only got 1 box each, and so far haven't gotten any new shipments.
So far I think we've gotten about 8 packs now, I think (most bought online). We got Isabelle in one of our last packs.
Please read my profile before sending a FC/Wi Fi Request.
I bought nearly 15 packs and finally got Isabelle in my last set of packs I received. I got maybe 5 duplicates of Special cards, so I had pretty good luck there. It was extremely satisfying to pull Isabella though. I was losing hope of getting her and even trading for her.
I bought a Japanese pack over the summer, and she was my first card. Woo Japanese Isabelle!
One of my gamestops got in a new shipment in so I picked up a pack, but I havent opened it yet. I'm waiting on a rainy day to do so. My friend is giving me a huge stack of cards because he doesn't want them anymore, I already had 54/100 cards, so I'm hoping this propels me closer to a full set.