Well, it's a bit technical, but I'll try to explain.
The game contains lots and lots of pixels, and these take a long time to produce because each pixel is the result of careful breeding between a pixie and an eel. That in itself is a difficulty, because the pixies show up to the dates with flowers and the eels just eat the flowers. But eventually the devs get enough pixel-eel babies (or "pixels" to use the technical term) that they can arrange them into what are known in the games industry as textures. These are large square sheets of cloth with a thin coating of cow dung spread on them. The coating makes the cloth sticky, and these textile manures (or "textures") can then be used to stick the pixels in place.
Of course, this has to be done by hand - and the pixels for each texture have to be carefully selected for color. When doing so, you also have to make sure that you don't put two aggressive pixels on the same texture in case they start fighting for dominance. Instead you have to choose dominant pixels that complement each other and work togethe to keep the texture in harmony. This is known in the trade as "alpha blending" the pixels.
With so many new textures in the game, you can see why this is taking so long.
