Johnny Depp's shtick hadn't gotten old, Mia Wasikowska became a bit of an 'It' girl after its release, so that helped with word of mouth, and Tim Burton's name carried a lot of weight with it. I think it was more the novelty of it being one of the first big budget live-action adaptations of a Disney classic. I reviewed the first movie quite positively, but it's a movie that I definitely need to watch again, but I didn't get a sense that people really loved this movie enough that it would justify a sequel. The problem comes in the fact that this wasn't a movie that anyone really wanted. That's not the worst drop in history, as there have been worse (Hot Tub Time Machine 2 being one of them, among many others), but it is a pretty big drop. The surprise comes in the fact that they waited 6 whole years to actually release it.

So, really, there should have been absolutely no surprise that a sequel was, eventually, gonna be commissioned.

A conservative estimate would be that the first movie must have made Disney something closer to $1.2 billion. I'm sure Depp's Mad Hatter character was really popular back in 2010, so merchandise featuring the character must have sold like hotcakes. And that's not even including the merchandising aspect of it. To give a little bit of perspective as to why this movie exists, the original Alice in Wonderland, Tim Burton's one at least, made Disney slightly over $1 billion worldwide.

Alice through the looking glass film