Any information you find on the internet, unless it is research conducted by a zoo or scientist, can not be used as factual. This is information I have gathered on the subject.
Hybrids are often sterile because the set of chromosomes from one species cannot pair during meiosis with the set of chromosomes from the other species. The Dobzhansky-Muller model theory was formed in the 1930s that suggests hybrid incompatibilities that cause thing such as death or sterility are caused by genes that have evolved from a common ancestor but diverged in each of the species. The best explanation for sterility in animals so far with limited studies is the unequal crossing over during meiosis that causes deletions and duplications of genetic material. If this always happens because the chromosomes do not align then the animal is sterile. If a critical gene is deleted or duplicated the animal may not survive to term or even to adulthood.
Just because you want to create a hybrid does not mean it is genetically possible. For example: Ligers are the offspring of a Tiger and a Lion. These are members of the Panthera genus. Since they belong to the same genus it has been possible to breed them. The lion and tiger may be subspecies of each other because they can mate to produce offspring. I would have to check this out but at least you get why hybrids happen even though they are sterile.