Why are there so few acids in the Willow Bark Peeling Mask?

Because it is stipulated by law. 
See the following text extracted from the Dutch commodities law: 
Skin peelings (Commodities Act Netherlands) 
With peelings, whether they fall under cosmetics or medical devices depends on the thickness of the skin layer they remove and the corresponding claim. If the deeper layers of the stratum corneum are exposed or even removed, this is a physiological effect and a product is not cosmetic.

Because Nannic does not believe in very acidic peels.   
Acid peelings such as trichloroacetic acid, Glycol peeling or phenol peeling, often cause injuries in depth down to the level of the papillary dermis.  Besides the fact that this is then a medical device, we at Nannic believe that such acid peels may give a good effect immediately after treatment but in the long term cause irreparable damage resulting in premature skin ageing.

The active ingredient is Willow bark extract (salicylic acid). 
The pH of the Willow Bark Peeling mask is: 5.7 with a tolerance of 0.3. 
"White willow bark" extract is Salicylic acid from the bark of the willow tree. 
The concentration is 2.5% or Willow bark which corresponds to a Salicylic acid concentration of 2%. 
This complies with current EU legislation. 
The product meets safety standards, which can also be deduced from the patch test conducted under occlusive patch