Two reasons: Quantum mechanics and dynamical instability.
Quantum mechanics means you CANNOT know the position and momentum (for our purposes, this = velocity) of a particle to infinite precision ('exactly') at the same time.
Dynamical instability ("chaos") means that even tiny differences in the initial condition of a chaotic system means even a small difference in the initial conditions will have hugh and series, unpredictable effects on the later course of the system over time.
Or, as I found elsewhere on the web: (obscenity warning)
Cheers!