several possibilities;
1 the EA revealed vastly more bugs than expected and the devs are focusing their primary effort on getting the core program working right.
2 doing a complete rework of the Mistlands biom is taking longer than expected.
3. trying to add extra content to the existing bioms to keep complaints about lack of updates to a reasonable minimum is taking developer time from an actual major update.

pick one to three of the above, or add your own.

The number of games sold has nothing to do with development speed. It is only a measure of how popular the game is, not how fast it will get developed.

Keep in mind that creating a game is more than just programming the core mechanics, you also have to make the frames, skeletons, textures, and animation scripts including combat mechanics for every creature and item. Then all that has to be play tested to ensure they are going to be balanced with the rest of the game. And that is just to get to Beta level content.

Its a lot more work than most people seem to think.