Maybe because I haven't created multiple accounts, thus not violating their rules? If they are so sure I have opened multiple accounts, why don't they tell me the usernames of "those" accounts?

first email after sending the documents https://prnt.sc/hif1w4
second email after saying I didn't have any other account and asked to at least tell me the usernames of the accounts I "supposedly" created https://prnt.sc/hif2al

Update: Sportsbet contacted me and gave me back my initial deposit of about 0.35BTC. However, my account is still frozen and the other 0.3BTC which I won is frozen.

Be careful, Sportsbet.io is stealing money from its customers without any proper explanation or reason. I created an account, made a deposit of about 0.35BTC, placed a few bets, won and had 0.654BTC in my account, a few days later I decided to withdraw my money but my withdrawal was declined, because I had to verify my identity. After I did so by sending all the required documents, sportsbet.io wrote me an email that my account will be frozen and they won't give me my money, as I had opened more than 1 account, however, I hadn't done so. I have only one account and asked them to explain in more details and got an answer that they wouldn't tell me anything more about my case. Moreover, they made me wait for about 10 days just to tell me they are going to steal my money from me. 
I couldn't imagine that sportsbet.io works money by stealing money from its customers. I guess this is the worst betting site, so be careful.