Many games are popular targets for griefers or DDOS attacks, not because the attacker gains anything of value from the attack (and many of them aren't actually hackers, they just download the tools written by others), but instead they do it solely for the perverse happiness they get from knowing that they ruined someone else's day / hour / minute.

Possibly you just haven't seen this problem yet because your game isn't high-profile enough for them to bother with and it doesn't provide enough amusement for them to get a kick out of it.