Gambling portrayed negatively because of the main reason gambling can have a negative effect to the person involved. And also I think gambling can lead a person to be addicted,greedy and some will commit crimes just to get a money to feed their addiction.
Gambling addiction is same like drugs/smoke addiction though and the result is always negative, i'm sure there are many lifes that become ruined because of gambling and also gambling is a sin on all religions. Even in my country, gambling is banned here.