I think its because you can easily doubling your money on gambling but the risk is you also can easily lose your money on gambling. I think gambling is makes more addicting when you win much money on gambling and it's make you want gambling more.
It's not easy to double your money its just something that can happen. It happens way less then losing happens that's for sure. When you win you want to go even further and make even more and this is the time you normally lose everything.