Because it's broken. OK, seriously, without actually being able to look at your car, it's impossible to say why *your* odometer isn't working, but I can tell you this: As I recall, the odometers on the 1997 Altima is mechanical. That being the case, if your car is an automatic, and it still shifts properly, and the speedometer works, then the problem is in the odometer itself. If your speedo doesn't work and your car is a manual, either, it could be in the instrument cluster or the speed sensor. From experience, the most common failure point is the instrument cluster.