Yes, it's a puzzle game, but anyone who's played it should know that it has a very steep learning curve that kicks-in very quickly. I'm not saying it's a bad thing, mind, but I think that people buying the game based on the "Casual" tag will be very quickly turned-off when they find out just how difficult this game gets at very early stages. Meanwhile, people specifically looking for a hard puzzle game will pass it up.
In short, we need more people tagging this game as "Hardcore" or "Difficult". "Casual" is the polar opposite of what this game offers.