why are some mezcal unsuitable for vegan ?

image found on Mexican Business Blog

Yes !

Pechuga is an old Mezcal technique wich varies from producer to producer and is most of the time only produced in tiny amounts for personal consumption. It is made by redistilling a finished mezcal with a mix of wild, seasonal fruits, often grains and always a raw, skinless chicken breast. As the chicken breast is suspended in the distillation process, the steam cooks through the chicken...

It is said that the chicken’s fat and proteins help soften the smokyness mezcal. The addition of wild fruit helps.

Let me quote Amy Steward to explain this very special and rare type of Mezcal further:
 
„There is just one ingredient that can make Mezcal different  (..) a chicken. Pechuga is a particular and wonderful version of Mezcal that includes wild local fruit added tot he distillation for just a hint of sweetness, and a whole raw chicken breast, skinned and washed, hung in the still as the vapors pass over it. The chicken is supposed to balance the sweetness of the fruit.”
 
This type of Mezcal is produced in tiny quantity and often only for local consumption.
Del Maguey released a (very expensive) Pechuga in the 2010s available only outside Mexico though. And lately another brand, Fidecio, just followed ("distilled with organic, estate-grown agave, wild apples, quince and their own farm-raised chicken breasts, is assertive and alluring with notes of tropical fruits, barbecue smoke and black pepper spice." according to Imbibe magazine.)