Which brings us to this questions:
Is it harder to control gambling or limit it without falling into excess than to stop gambling forever?
It is harder to not gambling forever than to set limit that is for the gambler who is already active in gambling but for those who have not gambled at all, it would be easier to not gamble forever.
Why is the effective strategy not moderation but absolute abstinence?
Both are effective strategies but depending on the situation one will be much more effective than the other one and vice versa. Like for a gambling addict it is best for him to have a n absolute abstinence to avoid the recurrence of gambling addiction, while for those who gamble regularly and is not addicted to gambling is best to have moderation because it would be easier for them to implement limit than absolute abstinence.