In a dictatorship only one person decides, the dictator (or, in a military dictatorship: the few people in power).
In a democratic country, a number of people needs to agree to this decision: actually the whole country through the representatives (if they don't agree with the decisions of the representatives, they won't choose them again).
And it's easier for one person, the dictator to decide on the war (especially as he will be well protected from harm) than for the whole country, especially since during the war the poorest suffer the most.