How come I have nasal polyps, but nobody else in the family has it?

Not always inherited. Nasal polyps can have different etiologies, not all of which are inherited from other family members (parents). Nasal polyps are often associated with chronic nasal symptoms (allergic or nonallergic rhinitis), but can often be associated with other disease states such as fungal sinusitis, cystic fibrosis, and aspirin-exacerbated respiratory disease ("samter's triad"). Ent eval recommended.
Have this checked. Nasal polyps are uncommon in childhood, but may be associated with certain diseases, such as allergies or cystic fibrosis. See your doctor.