There are three reasons:
1. The NaviBag bag is just too soft in my opinion. It lacks strength and a low-vibration view of the Navi allow.
2. The vewendete transparent window reflects - as far as my assessment - such that in sunlight not simply be seen.
3. the bag is placed on a tank bag used safely over you can look past the NaviBag on the road to my impression only from a height of about 185 cm again.
Of course these are all just highly subjective impressions. I will not stop anyone from trying it themselves.