Because every gamedev enthusiast should write tetris at least once!
To be honest this was just an excuse to refamiliarize myself with the canvas drawing API.
The first 18 commits were part of a 3-hour sprint in which most of the game mechanics were developed. You can watch a timelapse of this sprint here: http://vimeo.com/34310717
The game can be played here: http://dysfunction.github.com/tetris/