How can an allergic reaction only affect my nose?

Size matters... Often an allergic reaction is limited to the local site of exposure -- in the case of airborne pollens, dander, etc., this may be the nasal tissue. This is because anything larger than 10 microns (like pollen grains) will get caught in nose and that is where you are most likely to develop symptoms.
It may not be. You may be just more symptomatic in that area. There may be other symptoms you are not aware of. Itchy eyes, throat and palate? Itchy skin. Post nasal drainage? The nose is most commonly involved because the allergen is coming in direct contact with the nasal mucosa and causing the reaction.