I'm guessing this is in large part due to a generally low level of knowledge about how gambling (and particularly online gambling) works. At the time of my first encounter with Bitcoin gambling (Satoshi Mines) I was also quite ignorant but it took me ~10 minutes to figure out the probabilities. I'm still mystified why some people don't want to learn how gambling works and at the same time are willing to put a lot of money on their wrong assumptions. Let me know what you think.
It's not just ignorance, some people just have a tendency to think that laws don't apply to them, that they are special and have luck, and they surely will win. So, a person can understand that the odds are against them, and still gamble like crazy because of their superstition. I think this is far more common than not understanding the math behind gambling.