I don't know. It's just my personal experience that it's very rewarding to see a sign displaying a large amount of free-roaming birds, picking out one or two, and then doing your best to find them, even going through the region a few times to make sure you didn't miss anything.
I think they're kept in Asian regions mainly because of the namesake, keeping an Asiatic lion in an African region would be concidered strange and confusing. Along with that, you would keep an Asiatic lion in the Asian region because you don't have any other lion options for Asia.
If you want a lion in the Asian region, you can only pick the Asiatic lion. But if you want a crane in the European region you have a larger variety of choices that are more appropriate