Scientists don’t know for sure, but the most likely explanation is that rats probably would not survive an encounter with an infected carnivore, and so would not be around to transmit the disease to people.
It is also possible that rats may lack a means of transmitting the virus within the species, because of mouth shape or some other factor, and so do not become significant reservoirs for the disease. There is not much fighting within the species that would lead to the bites that would spread the infection.
The most common wild reservoirs of rabies are raccoons, skunks, bats, foxes, and coyotes. Domestic mammals, including cats, cattle, and dogs, can also get rabies.
But small rodents such as squirrels, mice, rats, hamsters, guinea pigs, gerbils, and chipmunks and lagomorphs such as rabbits are almost never found to be infected with rabies and have not been known to cause rabies among humans in the United States.
From 1985 to 1994, woodchucks (or groundhogs) accounted for 86 percent of the 368 reported cases of rabies among rodents.