Bestiality is quite rare, and difficult to prove without injury to the animal. It's actually technically "legal" in many places, just like cannibalism is technically "legal" in many places. The thing with Western law is that in order to create a new law there has to be a precedent. So Just like cannibalism didn't become illegal in Germany until that weird dude ate his boyfriend 5 years ago, obviously there was no need for a bestiality law in Sweden till now.