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.
I will attribute the negative portrayal of gambling to the individual lives that many gamblers or people seen to be gambling live out there which seem to not go down well with society because there have been many gamblers who are irresponsible at home, some do hard drugs, engage in thievery and other indecent behaviors.