Because a lower scores meant you combined fewer tiles to get to 2048, and that you won in fewer steps. ... User Ovolve over on Stackoverflow has just done that to produce just such a counter.

Instead of matching of 3, 6, 12, and so on, the goal is to match up as many tiles as possible to reach 2048 before the grid fills up. At first glance, Threes and 2048 look very much the same.