Because a ball that travels forward could of been generated by forward momentum, but not an attempt to pass it forward.
That's the issue with the officiating of forward passes.
If all balls that travelled forward were called forward, it would be a simpler process. But any physicist will tell you, if you are running at 20km an hour, and pass the ball in a direction (aimed backwards), there is a strong likelihood that it could travel forward as you are moving with considerable momentum. Many balls can be thrown backwards, but travel forwards due to this circumstance. Also - the ball is almost an oblong shape, so momentum plays an even further role here depending on what part of the ball the force is applied when a pass is executed (ie - if you "cup" the ball with your hands and pass, the pointy parts of the ball will carry more force, momentum and direction of how the pass travels).
It's quite technical, but the short of it is - this technology needs to determine whether a ball goes forward milliseconds after it has left the hands of a player. My worry is, some ball start out looking forward, but the shape of the ball and other factors come into play which make it go back... will that be applied here?
Either way - it's one of those things in league where we try to overcomplicated a process, and hope that technology will simplify it. Another failure in the pipeline.