baba-yaga

Just because it's not satisfying doesn't mean it's not an answer. Evolution is driven by random mutations in DNA. Sometimes they're beneficial, sometimes they're harmful. In those cases, natural selection determines whether the individuals with a given mutation will survive and pass on their genes. But sometimes, mutations are neither beneficial nor harmful, and which one beats out the other (if either does) is simply random chance. So many times, questions like yours are unanswerable.

Evolution, though directed by natural selection, is a random process. So sometimes the answer's just because that's how it happened.