Because there is no need to cover the face. The face is used for communication and identification. The face portrays your emotions and help others understand you better.
If the idea of a Niqaab is not to attract undue attention, it fails miserably on that count!!! Not only it will attract undue attention, it seems rude and creates apprehension fear and ridicule. Besides, where in the Qur'an it says wear a Niqaab?
Remember the issue is of Niqab and not the Hijab which no one is banning!!
In these days of terrorism, it also is a security issue, therefore undesirable.
Muslims are supposed to be balanced unless some believe there is no relation between religion and reason!!