They explained the change saying:
Waypoint and 343 Industries were always uncomfortable with the idea of taking stat-tracking offline, even though the technical and logistical problems presented made sense. The argument that it was a small and shrinking population didn't do much to quell the reaction of both Halo Wars fans and Halo FPS fans who were concerned about the future of other Halo titles, and we decided that the current plan of action was not in line with how we have always intended to support Halo games and the Halo franchise in general.
Though the player base may be small, they made the right call to save the stats. Their representation of the cumulative time and effort put in by those gamers lies as a cornerstone in the foundation of Waypoint. Seeing the switch flipped off for someone else's game, no matter how small the population, would have left many wondering when it might be their turn in that situation.