Why does my Nikkor AF-S DX VR 18-200mm's focus ring stop at 0.9m?

The most likely answer is that the lens has been damaged. I had a Nikkor lens (75-300/4.5-5.6) that had this same problem at one point. I sent it in for repair and Nikon advised that the lens had impact damage. It had most likely been dropped or fell onto a hard surface, which damaged the focusing helicoids.

The only way to know for sure would be to disassemble your lens (risky) or to send it away for service (costly).

Note that if the theory is right, you may not be getting optimum sharpness from the lens, either, as elements have possibly shifted slightly due to the impact.

A similar issue just happened to me with my Nikkor AF-S DX VR 18-200mm but instead of not getting below 0.9m, mine was a bit higher. My lens had been in storage for several years due to the fact that I basically moved to full-frame D600 and as such, I had really no need to use that lens along with my D7000. Today, I was forced to press the lens and the D7000 back into service and found the problem. The lens worked perfectly before it was placed into storage.

Luckily I was able to fix the problem rather easily. Recently, I was reading how the initial generation of the AFS 28-70mm f/2.8 lenses suffered from a similar problem where the lens would lose its autofocus ability due to a 'failure' in the lens' focusing motor. The failure was due to a slight bit of oxidation in the motor causing the mechanism to fail. The solution was to clean off enough of the oxidation so that the motor would move freely. Since I needed the lens right away, I didn't have time to get it repaired or disassemble it to clean it myself. So, I vigorously worked the focus ring back and forth between infinity and where the lens stopped focusing. Slowly but surely after about 5 minutes of working that focusing ring, the focusing range of the lens increased to its full range for both manual focus and autofocus.