As to why it's not more common in animals, Thorgaard says there are a variety of ideas as to why. Sometimes more genes are expressed from one of the parental sex chromosomes. Again, the proper ratio of chromosomes from each sex would be crucial. In support of these ideas is the observation that polyploid animal species tend to have sex chromosomes that are more similar than those of animals that do not. It may also be true that basic differences in how plants and animals grow are involved.
Animals have more deterministic growth patterns.
They have a multitude of complex systems, skeletal and nervous for example, that must be precisely integrated. Extra gene products from extra chromosomes could easily upset the balance needed for this integration. Current evidence suggests that a large percentage of all land plant species have originated not just once but several times. It's what we have. If they become distinct enough, they become a new species.