Actually, scientists are exploring this avenue.
Its just that cloning stupidly expensive. Do you have any idea how hard it is to clone one animal, let alone multiple species in enough numbers to breed and survive on their own? They would end up cloning very few animals of a species, and would have to keep them isolated. This would drain much resources that would be best used to help prevent causes of extinction. This is also ignoring the issues that would arise with genetic diversity, or lack-thereof.
Its an interesting idea, and scientists are looking into it, but at the moments its just unfeasible.
I believe the gaur is the only endangered animal to be cloned. And even then it was just one. And it died 2 days after birth.