Polyploidy, or why plants of different species can produce fertile offspring hybrids more frequently than animals?
While I do not know if there is a definitive answer to this question, I suspect that at least part of the answer can be found in the fact that animals typically have much more complex and delicate mechanical interactions in their bodies than plants.

From a mechanical perspective, plants are pretty simple. They move, but generally quite slowly and via very simple mechanical principles, and the interconnection between different tissues and organs is mostly via chemical and fluid flow.  Many plants are also highly flexible in their size and also change their size by orders of magnitude even after sexually mature.

Animals, by contrast, are often ferociously complex and intricate systems of joints and complex and highly differentiated organs. They also tend to have a well-defined typical size of mature individuals, which doesn't vary all that much compared to plants.

When you have a change in ploidy, a major effect is to multiply or divid the levels of gene expression, based on the change in number of copies.  Where that interacts with the processes of morphogenesis, it will thus often have radical effects on the size or growth patterns of the effected tissues or organs.  

With a more intricately connected and scale-sensitive system like an animal, it seems then unsurprising that such changes might be much more difficult to survive.