Why do only some people have allergies?

Good question: But do you want everybody to have allergies?! Like any other disease only few have it. As for the mechanism, simply put: there is a certain limit for the immune system tolerance, i.e. Ability to tolerate any material either external (none-self) or internal (self) as fine, once this tolerance is disrupted for a reason or another,allergy or hypersensitivity develops and the immune system over-reacts