All my important attacks have negative luck, why is that?

Luck tends to be around 0. At times it can go up to +30 or as low as -30.
Luck is blind. It doesn't know if an attack is cancelled, or just sent to an island quest, is a fake attack or the most important attack in your Grepolis campaign. It simply adds a random number (weighted to 0) to the equation. A person can have a streak of bad luck, but generally it will even out over time. We are sorry you had bad luck with this variable.