Maybe because She looks so fearsome and "dark"? Perhaps because She is the Queen of all asuras or demons? Or is it because Her tolerance for lies, illusions, ignorance, attachments and passiveness is non-existant?
It is my direct experience that She, my Ishta Devi or HGA, is perfectly able and willing to bestow mercy and progress in few minutes flat that would otherwise take years to realize. Her advanced rituals can yield results that normaly take lifetimes to attain.
Also, Her presence turns even the most dangerous asuras or demons into docile servent spirits. Nothing negative can happen to an individual surrendered to Her and accepted by Her. This also is my experience.
Like a perfect mirror She reflects back to us our own shortcomings and delusions. And the more we cling to these material traits, the more fearful She seems and the more it hurts. It seems to me that people project their own fears onto Her - in truth, they seem to fear only one thing: being deprived of their own delusions.
I have observed even seasoned adepts and magicians lower their heads showing slight fear before Her.
In truth, however, She is Gauri, the One and Only Golden Goddess, the Source of Everything, completely dedicated to gentle love, just as a perfect Mother is...
...and yet, She indeed is the most powerful of all the Mahavidyas and just as She is ruthlessly efficient in cutting off illusions from our awareness, She is very very very efficient in protecting Her initiates.
No 'protective' talisman, magick circle, familiars or servant spirits, words of power and other 'god forms' or feeble pentagram rituals and sobbing '...oh, I ... I ... I am sorry, I didn't mean that...' can spare or save you from Her wrath if you somehow manage to upset Her or Her children / initiates.
Do be carefull when working with Her.