Most of the gambling sites have mirror sites but as far as I know these are for the purpose of load balancing. For example, if there were no mirror sites, all the load would be on the same site but since the load can be divided by mirroring, it helps the gambling site to operate more efficiently.
Also if for some reason, one domain name / site is down, the other ones are readily available. But this is done by the old trusted sites or the ones that have established their business. You won't find such features with newly created online crypto casino.
There are many reason why sites like casinos have mirror sites such as security, marketing, traffic distribution, etc. But this one, is made only for japan users, you can observed it due to the site's language. For the question it was made for japan even though casinos/gambling are banned in japan, well, it doesn't differ for the casinos made for chinese as well, it's the same reason as there are still people from these countries who likes to gamble.