One reason cremation is used is to prevent disease. Tapeworm and some other diseases are highly infectious. If it is buried, the high level of bacteria can infect groundwater.
You mentioned only dogs, but there are cats, chickens, pigs, horses, etc. Small animals are easily cremated, but large animals are too big to fit and take a ton of energy to burn.
Some dog food companies use euthanized animals and road kill as ingredients in their product. Only greedy corporations and their stockholders prefer this method of disposal.
Environmentalists probably would dislike the former methods, but composting is a relatively safer method that consumes less energy and provides a high quality organic fertilizer that replaces chemical fertilizers. Composting is a great alternative.