Then why is not 3D Tetris like BlockOut or BlockOut2 growing in popularity ?
I've played 3D Tetris before (BlockOut is the name I guess), waay years ago. And it was ..... tough. Instead of one dimension rotation there are 3 dimension rotations. It's too much for me to optimize all rotation abilities to optimize the result.
And the blocks being displayed in 3D space is hard to see compared to 2D space. Sure if the blocks are placed properly you can see the blocks but if you messed up, blank spaces will be covered by blocks on top of them.
TLDR, 3D Tetris is complex and much harder to play than 2D Tetris.