Commercially bred Guppies are very un-hardy due to hundreds and hundreds of generations of overbreeding.
They are mass-produced in giant pools, so they don't get the greatest observational care.
They are also grown out in super high salinities, for disease controls and aid in osmoregulation due to them being a brackish fish by nature.
They are grown out from fry to salable adulthood in perfectly controlled conditions.
From the time they acclimate and re-acclimate to your home aquarium, they usually succumb to the stresses of mass production and lack of exposure to varied environments.
In other words, Guppies in the last few years have become incredibely un-hardy due to them being seen as mini cash cows for Florida and Asian breeders alike.