Nintendo hasn’t commented on why, exactly, it has had to pull Tetris, but it doesn’t take much to form an assumption. The game’s license is now owned by Ubisoft. It would appear the new year marks the expiry of Nintendo’s time with the brand, and they don’t want to renegotiate with Ubisoft.
At least, that’s the leading explanation right now. If you own a Nintendo 3DS and have been eyeballing the Game Boy version of Tetris on the eShop, buy it now before it’s too late. Come 2015, it’ll be gone.