There is also de possibility those who get scammed are lurkers without an account and little knowledge of their reputation and the trust system.
This is a possibility. However, most people who visit Bitcointalk are usually members of the site which is why the chances of lurkers signing up for 1xbit through this forum are pretty low if you think about it.
How is that possible that gambling site is getting popular being SCAM site and people willing to join the scam?
I agree. What @shasan mentioned doesn't really make a lot of sense. This forum is actually making them less popular over time.