kydogduster,
I would say coyotes have one litter of pups a year because the amount of time it takes them to raise the one litter. The whole process that it involves which is: the pair bonding/mating, denning/gestation/birth,and pup development take several months. Plus the pups often stay with the parents until late fall or early winter. As you can see there is no time for another litter of pups.