He died because it would have cost the pet shop too much money to call a vet to have him treated. So they let him die. If it is sick or does die, the pet shop will not give you back your money. They'll give you another puppy instead. That's how they make their money.
Puppies are cheap.
To irresponsible pet shops and to puppy mills, puppies are not lives. They either don't believe or don't care that dogs suffer pain, hunger, loneliness, fear. It doesn't fit into the bottom line, and all they care about is their profit margin. Far too many. Animals are dying for want of homes, but the puppy mills don't care.