Because Microsoft hasn't released enough details about the format to safely enable write access.
All native linux NTFS drivers have (I believe) been reversed engineered.
You have two options
1. Write a nice letter to Bill Gates explaining your problem and ask for the documentation for the format.
2. Try the captive NTFS drivers
http://www.jankratochvil.net/project/captive/ which use the original windows drivers in a wrapper, much like Ndiswrapper.
