Because of the very cold living conditions they'd have to endure.
cold
Rabbits were created to be a food source to other predators. If they didn't live, most carnivores would be rare or become extinct. Most predators depend mainly on the rabbit for food.
They should never hibernate they are to small to live of body and will most likely die if the conditions are to cold they will go in to hibernation which they should NEVER do
It offers a good home from most predators.
Sloths live high in the trees and they stay on branches where most predators can't follow them.
Cold climates
The sand rat (Psammomys obesus) like most animals that live in a desert environment do have predators such as raptors.
Most whale sharks, live in the deep ocean and feed on plankton. They live in the open ocean, so the large sharks will not be stranded in the shallows, and to avoid predators that live near the shore. Plankton can be found in the cold deep waters of the ocean, so that is where whale sharks tend to live.
Probably rain and cold damp conditions.
Access to food and safety from predators.
Seeming as though chihuahuas are so small, it is not advised. However, depending on the conditions you live in, its not impossible.