The reason is, people want to observe your casino first, hence they don't like when they get asked to complete KYC during registration.
I believe the reason people are mostly pissed off when crypto casinos request for KYC is usually because they value their privacy and anonymity. Not everyone likes to just give out their details simply because they wanna gamble, especially when it involves crypto transactions. Some people just wanna gamble and have fun without having to compromise their privacy or the safety of their personal details and this is very much understandable.